E. M, NELSON ON BINOCULARS. 



55 



Fi^.S. 



may be often seen when a photograph is projected by a lantern 

 on a screen. I have frequently noticed the boughs of trees 

 apparently stretching out of the screen into the room. 



But to return. It is pretty obvious that if we place precisely 

 similar images in each tube we cannot obtain the same kind of 

 stereoscopism as when dissimilar images are presented. We 

 nevertheless get a kind of stereoscopism which may be called a 

 bastard stereoscopism ; this can be seen in both the Abbe and 

 Powell binoculars when the whole of the eye-spots are used. 

 This bastard stereoscopism may also be repro- 

 duced in a single body by placing a stop over 

 the back of the objective with two lateral aper- 

 tures in it like Fig. 5. In passing I would 

 recommend all who take an interest in the 

 optical side of the microscope to provide them- 

 selves with a nose-piece adapter with a 

 slit in each side of it (Fig. 6). It is useful 

 for so many experiments, as it allows a 

 strip of paper with apertures or stops cut 

 in it to be placed over the back of the 

 objective. For lens testing also it is un- 

 equalled, as the paper strip in passing 

 through the slit causes no vibration. It can be used in con- 

 junction with my rotary nose-piece.* In lens testing a full cone 

 of light from the condenser should be used. It is important to 

 understand the principle of stereoscopic pictures as seen with 

 an ordinary stereoscope. Taking geometrical figures the two 

 truncated square pyramids are 

 suitable and well-known ex- 

 amples (Fig. 7). These very 

 dissimilar pictures combine 

 most perfectly in a common 

 stereoscope, and yield "ortho-" 

 or "pseudo- " stereoscopic effects 

 according to the way they are 

 between them perpendicularly 

 the ortho-stereoscopic image may be observed without instru- 

 mental aid ; the pseudo-stereoscopic image can also be seen by 

 viewing them through a square hole in a piece of card held 



placed, 

 to the 



By holding a card 

 plane of the paper 



* " Q. M. C. Journal," Vol. ii. (1885), p. 153 ; " E. M. " (1885), No. 1,042. 



