144 



Animal Micrology 



left outstanding form the defect known as a secondary spectrum. 

 In the apochromatic objectives (p. 146) three rays are brought to 

 one focus, leaving only a slight tertiary spectrum. 



NOMENCLATURE OR RATING OF OBJECTIVES AND OCULARS 



Oculars. Different makers, unfortunately, use different sys- 

 tems in marking their lenses to indicate relative powers of mag- 

 nification. In the case of lettering the system is wholly arbitrary ; 



the only rule is that the 

 nearer to A the letter is, 

 the lower the magnifica- 

 tion. When the objective 

 bears a figure it is usually 

 indicative of the magnify- 

 ing power of the part 

 marked. Thus a y 1 ^ inch 

 objective magnifies ap- 

 proximately 120 diamet- 

 ers ; a |- inch, 80 diameters ; 

 a -J inch, 20 diameters; a 

 1 inch, 10 diameters: a 2 

 inch, 5 diameters; and so 

 on. This means that an 

 objective which forms an 

 image 10 times the real 

 diameter of the object 

 itself, on a screen placed 

 10 inches (the conven- 

 tional distance of vision) 

 from its back lens, is rated 

 as a 1-inch objective. If it 

 FIG. 58.-The Zeiss niB stand with Diaphragm- formed an image only 5 



Carrier (B) and its Key (C). . .-, i i , ,- 



. times the real diameter c 



Shop, 324 Dearborn street, Chicago, 111. ^e object it would be a 2- 



inch objective, if 30 times, a J-inch objective, and so on. Such 

 magnification is termed the initial magnifying power of the 

 objective. 



