Ch. I] 



OCULARS FOR THE MICROSCOPE 



25 



Eye- Point 



Positive and negative oculars can be readily distinguished by in- 

 spection, as the ocular diaphragm, at the level where the real image of 

 the objective is formed, is between 

 the lenses of the negative t5^e, and 

 below all the ocular lenses of the 

 positive type (fig. 22, 23). 



§ 38. Huygenian ocular. — A 

 negative ocular devised by the 

 Dutch astronomer Huygens. This 

 is the most comrnon ocular used 

 on the microscope, and consists of 

 a plano-convex field-lens and a 

 similar, but higher power, eye-lens, 

 the convex surfaces of both facing 

 downward (fig. 23, 24). Theoret- 

 ically the focal length of the field- 

 lens is about three times that of 

 the eye-lens, but in practice the 

 ratio varies with the power, being 

 I to 1.5 or I to 2 with low powers 

 . and nearer i to 3 with the high 

 powers. The ocular diaphragm is 

 placed approximately at the focus 

 of the eye-lens. 



§ 39. Ramsden ocular. — This 

 is a positive ocular composed of 

 two plano-convex lenses with the 

 convex faces turned toward each 

 other, and so arranged that the real 

 image is formed below both lenses 

 (fig. 22 A), not between them, as 

 with the Huygenian ocular. In the 

 best modern forms of Ramsden oc- 

 ular the simple lejises are not used, but achromatic combinations (see 

 Ch. rX for further discussion). The Ramsden form is often used for 

 ocular micrometers (§ 243). 



Fig. 23. Low-power Huygenian 

 Ocular in Section. 



Axis The principal optic axis of 

 the ocular. 



FL Field-lens of the ocular. 



d, ri Diaphragm and real image 

 between the ocular lenses. 



EL Eye-lens of the ocular. 



Eye-Point The eye-point seen in 

 section and by looking down upon 

 the end of tlje ocular. 



