26 PRACTICAL HISTOLOGY. 



higher than that of air, the ray is not so much deflected 

 from the perpendicular at the upper surface of c in B, 

 as in A, and therefore it enters the lens. In the case of 

 very high powers, the full illumination of the object is a 

 most essential consideration ; hence the importance of the 

 immersion principle. For the same reason that water is 

 better than air, glycerine is better than water ; care, how- 

 ever, must be taken to employ pure glycerine. Glycerine 

 is, however, not so convenient as water, and for ordinary 

 purposes the latter is all that is required ; only distilled 

 water should be used, however, for the film upon the lens, 

 which ordinary water leaves when it dries, is by no means 

 unimportant. The immersion system is also advantageous 

 in another respect If Fig. 28 be compared with B, Fig. 

 30, it will be evident that, by diminishing the obliquity of 

 the rays, the water under the immersion lens must contri- 

 bute to the better definition of the object. The immersion 

 system should, however, be employed only for very high 

 powers, it being inconvenient in doing ordinary work. By 

 Ross and some other opticians, lenses are now made that 

 may be used either dry or immersed by altering an 

 adjustment. 



40. The Eye -Piece or Ocular. The eye-piece 

 commonly employed is that invented by Huyghens. It 

 consists of two plano-convex lenses placed as in Fig. 3 1 

 (E F). The rays from the objective would, if uninterrupted, 

 form an image at v v and r r. With the field-glass (F) 

 they are focalised at v f v and r /, and so produce a 

 smaller image capable of being embraced by the eye-glass 

 (E). Both lenses produce chromatic and spherical aberra- 

 tion, but these are reduced to a minimum as follows : If 

 the objective were perfectly achromatic, the light would be 

 decomposed by F, and the violet rays focalised nearer to F 

 than the red rays (Fig. 23). The focal length of E being, 

 as in the case of F, shorter for the violet than for the red, 

 an achromatic image would be impossible. This difficulty, 

 however, is overcome by over-correcting chromatic aberration 

 in the objective, so that the coloured light issuing from it 

 is focalised by the converging lens F in a manner the 



