OBJECT GLASSES AND EYEPIECES 



89 



Ingayepieces, 



instrument, and for this purpose a 17- or 15-mm. or a 10-mm. 

 eyepiece is required. A B-mm. eyepiece magnifying 50, and a 

 2-5-mm. magnifying 100, are made to order for special testing 

 purposes, and have their uses. 



Compensatina eyepieces are specially corrected to work with oomponaat- 

 apochromatic object glasses, and when of a higher power than 

 15 mm. are the best for use with achromatic object glasses. The 

 difference in performance of the Huyghenian and the compensating 

 eyepieces is not very marked. 



Huyghenian eyepieces consist of two plano-convex lenses, ^^f^^"*™ 

 one at each end of a tube, with a diaphragm between them. 

 It is an eyepiece that has many advantages for visual work, but 

 it is not the best for photography. It is also not quite so perfectly 

 corrected as the compensating eyepieces which are specially 

 made for work with apochromatic object glasses. The lower of 

 the two lenses is called the field lens, because it increases the size 

 of field while the upper one does the 

 magnifying. The two lenses are of such 

 powers and placed in such positions that 

 they are achromatic and give a fairly flat 

 field for visual purposes, but do not do 

 so for photography, where the microscope 

 has to be re-focussed in such a manner 

 that an actual image is formed behind 

 the eyepiece instead of a virtual image 

 projected in front of the observer's eye. 



The corrections of an eyepiece need 

 not be of so perfect a character as those 

 of an object glass, because the individual 



bundles of rays from each point of the object are very narrow 

 beams of light as they emerge from the eyepiece, and the defects 

 of an eyepiece are reduced in a similar manner to those of an 

 object glass when it is stopped down by a pinhole aperture. 

 The aperture which limits the size of the beams of light is not a 

 pinhole, but the same efiect is produced by the narrow angled 

 cones of light which come from the object glass. 



The magnifying power of the eyepiece is never very great 

 compared with that of the object glass, and it is only in those of 

 high power that the corrections are of such importance that the 

 extra quality of the compensating series are very noticeable, 

 except for photography or where entire freedom from colour is 

 essential. 



The so-called projection eyepieces are no better for projection 

 and photography than the compensating, and are far more difficult 

 to adjust. 



Eyepiece micrometers or plates of glass ruled with squares 

 or cross lines may be dropped upon the diaphragm in the tube 

 of the eyepiece by removing the cell holding the upper lens. By 



Fio. 90. 



