144 



Animal Micrology 



left outstanding form the defect known as a secondary spectmm. 

 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 J inch, 80 diameters; 

 a 4 inch, 20 diameters; a 



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 HIB Stand with Diaphragm- formed ail image only 5 

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



m c. t -fi times the real diameter ot 



Zciss is represented in America by The Scientific 

 Shop, 324 Dearborn street, Chicago, 111. ^ Q o hj ec f; ft WO uld be a 2- 



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

 magnification is termed the initial iii<i<jiiifi/iii(j jx>ircr of the 

 objective. 



