26 



OCULARS FOR THE MICROSCOPE 



[Ch. I 



§ 40. Compensating oculars. — These are either positive or nega- 

 tive oculars chromatically overcorrected to compensate for and 

 correct the residual color defects in the extra-axial portion of the 



visual field due to the non- 

 achromatic front lens of the 

 objective (fig. 22 B). They are 

 regularly used with apochro- 

 matic objectives, and may be 

 used to advantage with high- 

 angled objectives of the ordinary 

 type (see further in Ch. IX). 



Power of Oculars 

 As oculars of all kinds are 

 made in different magnifying 

 powers, there is needed some 

 form of designation. Many 

 different ways of designation 

 have been used, as lettering, 

 numbering, giving the equiva- 

 lent focus, etc. 



§ 41. Lettering or number- 

 ing. — This is a purely arbitrary 

 form of designation, but prac- 

 tically all of the opticians 

 adopted the rule that the lower 

 power should be designated by 

 the first letter of the alphabet, 

 or No. 1, and that the succeed- 

 ing letters or numerals should 

 indicate progressive increase in power, although there was no general 

 agreement as to the exact amount of that increase. Occasionally 

 one still meets with oculars lettered A, B, C, D, or numbered 1, 2, 3, 

 4, etc. In any given make of ocular one has simply to remember 

 that the earlier the letter or the smaller the number, the lower is the 

 power. 



Fig. 24. 



High-power Huygenian 

 Ocular. 



Axis The principal optic axis. 



FL Field-lens. 



d, ri The diaphragm and real image 

 between the ocular lenses. 



EL Eye-lens. 



Eye-point The eye-point in section 

 and face view, looking down upon the 

 upper end of the ocular. 



