112 Heisch, on Nachefs Stereo-Pseudoscopic Microscope. 



oi" 45°, the reflecting surface c, d being just large enough to 

 cover half the aperture of the objective. 



The glass is so mounted that it can be pushed half way 

 across the objective^ in Avhich case the reflecting surface c, d 

 will be opposed to the left-hand half of the objective, in- 

 stead of to the right, as in the figure, g, i, A is a reflecting 

 prism, the face, y, i, being parallel to e,f, and g, h, at right 

 angles to rays entering the prism perpendicular to g, i, and 

 reflected from i, h. One body of the microscope is fixed so 

 as to receive the rays r, r, which pass from that half of the 

 object-glass not opjDOsed to the reflecting surface c, d. The 

 other body is so placed as to receive the rays r', r' , which 

 have been reflected from c, d, and i, h. When c, d, e, /is in 

 the position represented in the figure, the effect is stereo- 

 scopic ; when it is pushed so that the rays from the other 

 half of the object-glass are reflected, the left-hand image is 

 j)resented to the right eye, and the right-hand to the left 

 eye, and the effect is, of course, pseudoscopic. 



The disadvantages to be overcome are these : 



1. The unreflected image is seen only through the thick 

 piece of glass c, d, e, f, and though, if this be very perfectly 

 worked, the loss in definition is not great, it is still quite 

 perceptible. 



2. Owing to its large size, the glass c, d, e, f can never 

 be completely removed from the object-glass, so the instru- 

 ment cannot be used as a uniocular microscope. 



"3. From the same cause, the prism y, i, h must be so far 

 from c, d, e, f that the bodies of the microscope must be 

 nearly parallel, which prevents the possibility of using the 

 draw-tubes as a means of adjustment for the difference in the 

 width of different persons' eyes, which adjustment is obtained 

 by making g, i, h, together with the body over it, move in a 

 horizontal direction nearer to or farther from c, d, e, f. 

 This arrangement gives rise to two inconveniences : — First. 

 If the eyepieces are so made that both images shall be in 

 focus when g, i, h is in any given position, the reflected 

 image is thrown quite out of focus as soon as it is moved. 

 Second. It is difficult to make a fitting to carry the j)i'isn» 

 and the body of the microscope which shall not become loose 

 by wear, in which case the instrument is at once out of 

 adjustment. 



To remedy this defect, I first retluce the glass c, d, e, f 

 to a simj^le reflecting prism by cutting it down the dotted 

 line from d. The direct image is now seen without the in- 

 tervention of any glass ; by appropriate mounting, the prism 

 may still be moved from one side of the object-glass to the 



