272 SUMMARY OF CURRENT RESEARCHES RELATING TO 



eye-pieces, the halves of which are actually covered with diaphragms, 

 for when the inner halves are cut off the tubes naturally require to be 

 racked down to diminish the separation of the eye-pieces, and in the 

 converse case to be racked up ; Dr. Mercer also satisfied himself by 

 experiment as to the validity of his deductions by observing sugar 

 pills pushed half-way through holes in black cards, the pills being 

 marked with cross marks in pencil to increase the effect. They could 

 be made to apj^ear convex, concave, or flat, according to the position 

 of separation of the draw-tubes. 



We have, for simplicity, referred to the covering up of both halves 

 of the eye-pieces, but it is not of course necessary to cover up more 

 than one* 



In order to obtain the best stereoscopic effect the halves (or one 

 of the halves) of the eye-pieces of the Powell and Lealand or other 

 similar binocular arrangements should be actually shaded by dia- 

 phragms so as to aid in properly centering the piipils, but Dr. Mercer's 

 object is to show that the effects observed with ordinary eye-jiieces 

 are explicable upon proper theoretical principles, and so to relieve 

 those observers who have insisted upon the existence of true ortho- 

 scopic effects in such cases, from the reproach which has un- 

 justifiably attached to them on account of their supposed abnormal 

 and unscientific development of a power of drawing upon their 

 imagination. 



[The Bibliography for the period intervening between tliat contained in the 

 Journal of October 1880 and the end of 1881, will be found in the Appendix to 

 the next volume.] 



Abbe's Expeiiinents on the Diffraction Theory of Microscopical Vision. 

 [General Remarks.] 



Jo'irn. of Sci., TV. (1882) pp. 118-9. 

 Acme Microscopes. Amer. Natural., XVI. (1882) p. 261. 



American Society of Microscopists. 



[Review of Proceedings for 1881, and remarks on the meeting at Elmira 

 for 1882.] 



The Microscope, I. (1882) pp. 175-7. 

 Angular Aperture. 



[Letter by ' Akakia,' describing Dr. Robinson's method of measurement.] 



Engl. Mech., XXXIV. (1882) pp. 454-5. 

 Beowneli,, J. T. — A nuich-needed stop. 



[Suggestion for a " thumb-.screw " to prevent Microscopes at Soiie'es being 

 focussed too low to the injury of tlie slides.] 



Amcr. Mon. Micr. Journ., III. (1882) p. 39. 

 Bvlloch's New " Congress" Stand. 



Amer. Mon. Micr. Journ., III. (1882) pp. 9-l.S (2 figs.). 

 Carlisle Microscopical Society — Inaugural Address by the President, Canon 

 Carr. Nnrtk. Microscopist, U. (1882) pp. 17-19. 



Cakr, E.— 5'ce Carlisle. 

 Cheap Microscopes. 



[Letter by C, advocating the encouragement of their purchase and 

 display, and further discussion by Welborn, G., Ollard, J. A., Cooper, 

 C. C, F., J., E. Holmes, A., E. C, and Medehanstade.] 



Engl. Mech., XXXIV. (1882) pp. 470, 495-6, 520-1, 545. 

 Cox, J. D. — Prof. Rogers' Micrometers. 



Amer. Mon. Micr. Journ., III. (1882) pp. 23-5. 



* See this Journal, i. (1881) p. 211, Fig. 38. 



