ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 313 



continuous with, or slightly below, the stage of the Microscope. 

 Exact approximation is not necessary. When properly adjusted, the 

 rest is perfectly firm and steady. When portability is not required, 

 the hinges and hooks may be dispensed with, and the wooden strips 

 fastened together with glue and brads. 



Astigmatic Eye-piece.* — Mr. E. Gundlach discusses the nature 

 of astigmatism and its interference with the perfect use of the eye, as 

 well as the relation of the astigmatic eye to the use of optical in- 

 struments and the injurious effects of astigmatism on microscopic 

 observations. 



As a remedy he proposes the use of an eye-piece of an asymmetric 

 form, so as to just neutralize the asymmetry of the crystalline lens of 

 the eye. This can best be done by making the outer surface cylin- 

 drical instead of spherical or plane. It may be made either concave 

 or convex as the requirements of the case may demand. The eye- 

 piece must be constructed with special regard to the purpose, so as 

 to place the asymmetric surface in such close proximity to the eye 

 that no perceptible secondary distortion is produced by the oblique 

 direction of the eye towards the edge of the field, and at the same 

 time the prismatic colours dispersed in the direction of the astigmatic 

 distortion must be neutralized. 



Mr. Gundlach intends to construct such eye- pieces, and expects to 

 start with a 1 in. To enable the applicant, for this special purpose 

 at least, to be his own examiner for astigmatism, he intends to furnish 

 with the eye-piece three eye-glasses, alike in mounting but different 

 in the degree of asymmetry, for selection ; the difference being such 

 as to practically approach both limits of common astigmatism. 

 The one of the three lenses nearest in asymmetry to that of the eye 

 will correct the astigmatism to an undisturbing minimum. The 

 observer will then have to test all the lenses, beginning with the 

 weakest, on a suitable object, slowly revolving the eye-piece until 

 its best position is found. Mark this position, and do the same 

 thing with the other lenses. After this, compare the action of the 

 lenses, each in its best position, to find the one best fitted for the 

 eye. Of course the eye-piece, or rather its asymmetric eye-lens, 

 must then always be used in the same position to the astigmatic 

 axis. 



Dr. J. K. Stockwell considers * that while Mr. Gundlach's plan 

 is quite feasible and very excellent in optical results, there are several 

 serious objections that may be mentioned. 



The first, and perhaps most tenable one, is the fact that while the 

 eye-piece would perfectly suit the person for whom it was made, — 

 one eye at least — not another one in several thousand could use it, 

 unless it was so constructed as to admit of having the eye-lens, the 

 asymmetric part, readily removed and replaced by a symmetrical one, 

 and the optical results would not be commensurate with the trouble 

 and expense involved. 



Complicated combinations of spherical and cylindrical lenses, 



♦ The Microscope, vi. (1886) pp. 1-4. t Ibid., pp. 29-32. 



