22 THE USE OF THE MICROSCOPE 



RULKS UlSKEOARDKD RuLES REGARDED 



3. Blurring increasing to the margin. Sliarp to the edge. 



4. Discomfort from the unoccupied No marked discomfort from the 



eye. unused eye. 



5. Side Hght in the observing eye. Side light shut out by blackened 



margin around lens. 



6. Right eye usually used alone. Eyes alternated, the position of the 



lens being a reminder. 



7. Eyes soon fatigued. Work may be carried on for hours 



without fatigue. 



Binocular Magnifiers. — The low-power magnifier, con- 

 sisting of a single corrected lens on a fixed stand, or any- 

 other fixed monocular magnifier, is well replaced, with 

 marked gain, by a good form of low-power binocular 

 magnifier. Binocular vision aids, in ways which will be 

 discussed later, in long-continued work. Stereoscopic 

 relief, which is shown by some binoculars, gives a sense 

 of reality not attainable by one eye alone. Whether 

 stereoscopic effect is necessary for most scientific work 

 seems doubtful. But of the advantage of the use of two 

 eyes, w^ith or without stereoscopic effect, there can be little 

 doubt. 



If two corrected lenses, with magnification below or up to 

 3 times, are placed before the two eyes, there is excessive 

 convergence of the optic axes, because the lenses have 

 to be held as close as their focal lengths from the object. 

 This excessive convergence may be lessened by the use of 

 two rhombohedral prisms, giving two total reflections 

 each. These magnifiers are worn like spectacles (27). 

 The optical effect of the prisms on the aberrations is that of 

 a plate of glass with parallel sides, as thick as the path of 

 light in the prism. Similar prisms can be used in 

 the monobjective binocular to vary the distance between 

 the eyepieces (Zeiss). 



An excellent spectacle magnifier (Zeiss), with a magnifi- 

 cation of 2 and a working distance of 20 centimeters, 

 consists of two special Galilean combinations (miniature tel- 

 escopes) properly convergent. The writer can testify that 

 these glasses are well fitted for dissecting, because of their 

 lightness and their optical corrections (Fig. 5). 



