24 MICROSCOPE AND ACCESSORIES {CH. I 



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

 Microspectroscope, Ch. VI. Statiroscopic Ocular ; Fr. Oculaire Stauroscopique. 

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

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

 see 39. 



g 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 lo. 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, 8, 12, 

 18, are positive. The numbers 2, 4, 6, 8, 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). 



$ 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 readily 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 may also be used, with ordinary achromatic 

 objectives of large numerical aperture. 



