Technical Field [001] This invention relates to a method to obtain optically pure Bisphenol A. Background Art [002] The USP Patent No. 5315042 describes a continuous process to manufacture bisphenol A, by contacting phenol with acetone in the presence of an acid catalyst, at increased flowrates through the catalyst, the said flowrates being sufficient to reduce the conversion of acetone whereby the content of acetone is kept at a high level, accounting for a high reaction rate and a suitable increase in the yield of bisphenol A, appropriate separation of the resulting bisphenol A before acetone is depleted. Owing to this, the residence time of bisphenol A in the reaction zone is reduced and the content of byproducts affecting the coloration of bisphenol A is reduced as well. The method has the disadvantage of being too energy consuming, which results from the low conversion of acetone and adequately low (10-13 %) increase in the concentration of bisphenol A in the reaction, whereby the amount of unreacted raw materials for pr ocessing and for regeneration per 1 ton of product is very high. [003] The International Patent No. WO/19302 of China Petro-Chemical-Corp. describes a method to obtain bisphenol A with high purity, according to which conversion from approximately 4- to approximately 12-fold ratio by mole of phenol to acetone in the presence of a modified ion-exchange resin catalyst is carried out in an essentially vertical, multistage suspension stripping apparatus. The entire phenol required for the reaction of condensation with acetone is fed to the tower-reactor at a point over the top plate, part of the entire acetone required is fed to the plate below the top one, re¬ spectively, and to some or all of the plates located at lower levels, whereas an inert gas stream is made to flow upwards through a catalytic chamber, thus forming a liquid- solid suspension, to strip off water from the reaction mixture. Technically, the method is highly complex and impracticable. [004] The Polish Patent Application P - 347705 describes a process carried out with the use of a double-zone catalytic bed, combined with post-reaction mixture take-off methods that are appropriate for such zones, and with keeping sufficiently high con¬ centrations of water and appropriate temperatures in the catalytic bed, as well as with appropriate purification of the resulting product. Moreover, the process is characterized by low increase [the concentration of] of bisphenol A in the reaction, due to which the consumption of energy in the process is too high. Disclosure of Invention Technical Problem [005] It was the purpose of the invention to develop a method to manufacture optically pure bisphenol A with selectivities above 95 % so as to obtain an increase in [the con¬ centration of] bisphenol A of at least 23 % in the reaction unit. Technical Solution [006] In the method of the invention, in Step I of the condensation reaction the reactants are pre-reacted to obtain an increase in the concentration of bisphenol A by 2 - 15 %, preferably by 3 - 5 %, while in Step II the reaction is carried out so as to obtain a difference in the concentrations of water between the inlet and outlet of the reactor in the reaction Step II in the range 0.1 - 0.5 %, preferably 0.1 - 0.2 %, while the difference in the concentrations of acetone between the inlet and outlet of the Step II reactor should not exceed 2%, preferably 1 %, and the difference in the concentrations of bisphenol A at the inlet and outlet of the reactor ought to be 1 - 5 %, preferably 1 - 2 %, while the difference in temperatures between the inlet and outlet of the Step II reactor ought not to exceed 15°C, preferably 50C. [007] Part of reaction water is removed from the reaction mixture in a continuous manner in the condensation Step II. [008] The post-reaction mixture resulting from the Step II and containing not less than 15 %, preferably not less than 23 % of bisphenol A, is sent to a distillation system to distill off a fraction that contains acetone, water and phenol, obtaining a residue in the form of crude bisphenol A, containing not more than 5 %, preferably not more than 1 % of phenol, is sent to separation by distillation or to melt fractional crystallization, where optically pure bisphenol A and a residue that contains bisphenol A, its isomers and other byproducts are obtained. [009] The post-reaction mixture from the condensation reaction Step II is sent to a suspension crystallization phase that proceeds either in one or in two steps, to obtain a crystalline bisphenol A-phenol adduct in the form of a suspension, which is subjected to filtration or centrifugation to separate the adduct which is then fed to a distillation separation system in order to separate bisphenol A. Post-crystallization liquor after water has been distilled off, is fed to the reaction Step II or to a distillation separation phase. [010] Part of or the entire phenolic stream used in the crystallization (recrystallization) Step II is contacted with an acid ion-exchange resin. [011] The post-reaction mixture from the condensation reaction Step I, prior to being sent to the reaction Step II, is used for washing the crystalline adduct during filtration or centrifugation after suspension crystallization. [012] The residue from the distillation separation or from the fractional crystallization is diluted with the stream of phenol and is subjected to static crystallization, to obtain a bisphenol A-phenol adduct, which is sent to the distillation system together with the post-reaction mixture stream from the condensation reaction Step II. The residue from the static crystallization is carried outside the process after phenol has been distilled off. [013] The residue from the static crystallization is distilled to distill off phenol and is then thermally decomposed in the presence of basic catalysts. The distillate resulting from the distillation is diluted with a stream of phenol and contacted with a cation exchanger, whereas the rearrangement reaction product is sent to the static crys¬ tallization. [014] The residue from the distillation separation or from the melt fractional crys¬ tallization is diluted with a stream of phenol and subjected to isomerization by contacting it with an acid ion-exchange resin at a temperature in the range 55 - 950C, preferably in the range 70 - 850C. [015] The isomerization product is fed to the static crystallization phase or the whole or part of it is sent to the distillation system. [016] The streams containing byproducts and bisphenol A, such as the residue from the distillation separation or from the melt fractional crystallization, are thermally decomposed in the presence of basic catalysts, and the distillate resulting from the de¬ composition is diluted with a stream of phenol and is then contacted with a macroporous cation exchanger while the rearrangement reaction product is recirculated to the process. [017] From 25 % to 55 % of the Step I reactor and from 30 % to 95 % of the Step II reactor are filled with the catalyst. [018] The bottom part of the Step II reactor comprises an ion-exchange resin that does not contain any active promoter, whereas the top of the Step II reactor comprises an ion- exchange resin with an active promoter. [019] The top of the reactor comprises a layer of beads such that their diameter is from 0.2 to 1.8 mm and which are made of a plastic that is resistant to the reaction en¬ vironment. [020] The catalyst used is a mix comprising a cation exchanger with a promoter and a cation exchanger without any active promoter. [021] The crosslinking degree of the cation exchanger without any active promoter is not more than 4 % DVB while the crosslinking degree of the cation exchanger with a promoter is in the range 2 - 5 % DVB. The crosslinking degree of the cation exchanger without any active promoter is 2 - 5 % DVB while the crosslinking degree of the cation exchanger with a promoter is not more than 4 % DVB. [022] The catalyst with the promoter is a sulfonated styrene-divinylbenzene copolymer in which 10 % - 99 %, most preferably 50 - 80 %, of sulphonic groups are neutralized by the promoter. [023] The Step II reactors and, optionally, the Step I reactors are divided each into at least two reaction sections by at least three sets of slot nozzles, located at various heights of the reactors. [024] From one to six Step II reactors fall to every Step I reactor. Advantageous Effects [025] The method of the invention enables optically pure bisphenol A to be obtained with selectivities up to 98 % and, due to unusually high (20 - 25 %) increase in the con¬ centration of bisphenol A in the reaction, the amount of unreacted raw materials for distillation and processing is much smaller, due to which the consumption of energy in the process is very low. Best Mode for Carrying Out the Invention [026] Example 1. [027] The condensation process is carried out in two steps: Step I is carried out in a 20-m3 reactor filled with a fixed bed of Purolite CT 122 catalyst with 15 % of sulfonic groups neutralized with 2,2-dimethylthiazolidine. The catalytic bed volume is 6.5 m3. [028] The reactor is fed upwards with a mixture of phenol and acetone in the ratio of 20 ÷ 1 by mole. The reactor is kept at a temperature in the range 55 - 570C. The post- reaction mixture from the reactor Step I, containing 5.2 % of bisphenol A and 0.2 % of byproducts, is divided in two equal streams and is sent to two Step II reactors with a capacity of 85 m3 each and containing a fixed bed of 40 m3 of CT 122 cation exchanger with 23 % of sulphonic groups neutralized with 2,2-dimethylthiazolidine. The reactor has 3 sets of filtration-injection slot nozzles in the bottom, middle and top sections. The reaction mixture circulates between the bottom and middle sets of slot nozzles. A stream containing water, acetone and phenol is distilled off from the reaction mixture using an evaporator operated at a 100-mmHg vacuum, at 970C. The reaction parameters are as follows: temperature is 6O0C at the inlet to the reactor and 720C at the outlet. Water concentration is 1.2 % at the inlet and 1.4 % at the outlet, concentration of acetone is 3.2 % at the inlet and 2.1 % at the outlet, concentration of bisphenol A is 23.1 % at the inlet and 25.4% at the outlet. The post-reaction mixture that contains 25.4 % of bisphenol A and 1.2 % of byproducts is fed to a distillation system to obtain crude bisphenol A, containing 1.2 % of phenol and 4.3 % of byproducts. The crude bisphenol A, of which the purity is approximately 95 %, is distilled at approx. 1 mmHg vacuum, to obtain optical bisphenol A of which the purity is 99.98 % and the color of a molten product is 5 APHA units, and a residue containing a mixture of bisphenol A and byproducts . [029] The bisphenol A distillation residue is diluted with a stream of phenol and subjected to static crystallization, whereby an adduct of bisphenol A with phenol comprising: 62 % of bisphenol A, 37.9 % of phenol, 0.1 % of byproducts is obtained. The molten adduct is fed to distillation together with a stream of the post-reaction mixture from the condensation Step II. [030] The static crystallization residue is subjected to distillation to distill off phenol and the resulting residue is thermally decomposed in the presence of 0.3 % of NaOH, at 24O0C, at a vacuum of 10 mmHg. The resulting distillate is diluted with phenol at the ratio of 2 ÷ 1 and is contacted at 720C with 5 m3 of Purolite CT 175 macroporous cation exchanger. The resulting rearrangement product is recycled to the static crys¬ tallization phase. Mode for the Invention [031] Example 2. [032] A crude bisphenol A, obtained as described in Example 1, is sent to a melt fractional crystallization phase, producing optical bisphenol A of which the purity is 99.98 % and the color of a molten product is 5 APHA units, and a residue that contains bisphenol A and byproducts, which is sent to thermal decomposition, where at 2450C in the presence of 0.4 % of NaHCO , at the vacuum of 15 mmHg, a mixture containing principally phenol, p-isopropenylphenyl, its linear dimers and oligomers is obtained. [033] The mixture referred to above is diluted with a stream of phenol at the ratio of 2.5 : 1 and is contacted with 5 m3 of Purolite CT 175 macroporous cation exchanger at 75° C. The rearrangement product is fed to distillation along with the post-reaction mixture from the reaction Step II. [034] Example 3 [035] The melt fractional crystallization residue is diluted with a stream of phenol at the ratio of 1.5 : 1 and is contacted at 780C with 10 m of Purolite CT 175 macroporous cation exchanger to effect isomerization and rearrangement of byproducts, principally the 2,4-isomer, towards bisphenol A. The resulting product is sent to static crys¬ tallization and continued to be processed as described in Example 1.