E. M. NELSON ON BINOCULARS. 61 



*' there is no true perspective difference of the images by 

 different portions of the apertures, because the microscopic 

 image does not admit of a perspective shortening of the lines, 

 which are oblique to the direction of the delineating pencils." 

 We must remember, however, that there is a difference in the 

 amount of lateral displacement of the images by different por- 

 tions of the apertures, and also that the difference between 

 lateral or parallactic displacement and perspective projection is 

 wholly unrecognizable in the microscope. Prof. Abbe has done 

 excellent service in combatting the absurd idea of " all round 

 vision." Who the author of " all round vision " was, I know 

 not, but I do not think that theory can be fastened on the late 

 Dr. Carpenter. It is true that he uses the word perspective in 

 connection with the microscope image, but just before he used 

 it in the same careless and incorrect manner when speaking of 

 microscopic pictures. It seems to me after careful study of his 

 article * that he did not intend the word perspective to carry 

 with it the conception of an all round vision. 



Finally his theory with regard to the 40*^ limit of aperture for 

 perfect stereoscopic effects cannot be maintained, because the 

 very slight depth of the microscope image requires a large 

 amount of lateral displacement. I hold that when a certain 

 stereoscopic effect has been obtained with a lens of a certain 

 aperture, if the power be increased by means of a deeper eye- 

 piece, the stereoscopism will remain unaltered, because the 

 lateral displacement is magnified equally with the object. Thus 

 if a series of spheres diminishing in size were examined with a 

 certain lens under varying powers, whatever might be the 

 character of the stereoscopism of one of the larger spheres with 

 a low power, i.e., whether the effect was under, sufficient or 

 hyper-stereoscopic, a smaller sphere under a higher power 

 would exhibit the same degree of stereoscopism, the illumina- 

 tion remaining the same in both cases. 



The best stereoscopic effects are obtained when a stop 

 (Fig. 4) is used at the back of the objective and duplex illumina- 

 tion employed. Great caution is necessary, so that the effect 

 shall be sufficient, and neither over nor underdone. The best 

 plan when the lens has been selected is to try various card- 

 board stops, and when the proper size is found, have them made 



* Carpenter, 5th Edition, 1875, p. 70. 



