24 



MICROSCOPE AND ACCESSORIES 



[CM. I 



or spectroscopic Ocular ; Fr. Oculaire spectroscopique ; Ger, Spectral-Okular, see 

 Microspectroscope, Ch. Vl.—Stauroscopic Ocular ; Fr. Oculaire Stauroscopique. 

 Ger. Stauroskop-Okular. An ocular with a Bertraud's quartz plate for mineralog- 

 ical purposes. — Working Ocular; Fr. Oculaire de travail; Ger. Arbeits-Okular, 

 see 'i 39. 



I 38. Huygenian Ocular — A negative ocular designed by Huygens for the 

 telescope, but adapted also to the microscope. It is the one now most commonly 

 employed. It consists of a field-lens or collective (Fig. 30), aiding the objective 

 in forming the real image, and an eye-lens which magnifies the real image. While 



Ocular Ho 2 



Fig. 31. Compensating Oculars of Zeiss, with section removed to show the con- 

 struction. The line A- A is at the level of the upper end of the tube of the micro- 

 scope while B-B represents the lower focal points. It will be seen that the mount- 

 ing is so arranged that the lower focal points in all are in the same plane and 

 therefore the microscope remains in focus upon changing oculars. ( The oculars are 

 par-focal ). The lower oculars, 2, 4 and 6 are negative, and the higher ones, S, 12, 

 iS, arc positive. The numbers 2, /, 6, S, 12, 18, indicate the magnification of the 

 ocular. From Zeiss' Catalog. ) 



the field-lens aids the objective in the formation of the real, inverted image, and 

 increases the field of view, it also combines with the eye-lens in rendering the 

 image achromatic. (See £46). 



2 39. Compensating Oculars. — These are oculars specially constructed for 

 use with the apochromatic objectives. They compensate for aberrations outside 

 the axis which could not be so readil\- eliminated in the objective itself. An ocu- 

 lar of this kind, magnifying but twice, is made for use with high powers, for the 

 sake of the large field in finding objects; it is called a searching ocular; those 

 ordinarily used for observation are in contradistinction called working oculars. 

 Part of the compensating oculars are positive and part negative. ( Fig. 31. ) 



# 40. Projection Oculars. — These are oculars especially designed for project- 

 ing a microscopic image on the screen for class demonstrations, or for photo- 

 graphing with the microscope. While they are specially adapted for use with 

 apochromatic objectives, they mav also be used, with ordinary achromatic 

 objectives of large numerical aperture. 



