EYEPIECES 



329 



three fourths of its component lenses. It is nearly achromatic and gives 

 a flat field with little aberration. 



In the best modern Ramsden oculars the simple lenses are replaced 

 by achromatic combinations. They are primarily used in the eyepieces 

 of micrometer microscopes, in connection with cross hairs attached to 

 movable carriages mounted on precision screws, to measure details in 

 an image. 



EL 

 EL 6^.-.-I"7? ?/i =1 



Simple 

 (a) 



Modern 



Fig. VIII-11. (a) Ramsden eyepiece. Positive ocular. PP plane of movable 

 cross hairs. EL, eyelens. FL, field lens. F, equivalent focal length. (6) The 

 modern ocular shown with achromatic combination to view image in plane of cross 

 hairs PP. (c) Huygenian eyepiece. Negative ocular. EL, eyelens; FL, field 

 lens; d, diaphragm; F, equivalent focal length. 



The negative ocular (Huygenian eyepiece) is the most common eye- 

 piece used in the microscope. It is effectively achromatic if both 

 lenses have the same index of refraction, and it has less distortion and 

 aberration than a single lens. It consists of two convergent lenses, 

 Fig. VIII-11, an eye lens (EL) and a field lens (FL), whose focal lengths 

 are in the ratio 1 to 3. The distance between them is twice the shorter 

 focal length. In this eyepiece the rays from the objective must pass 

 through the field lens before coming to a focus in the plane of the dia- 

 phragm inserted between the lenses. It is equivalent to a single lens 

 whose focal length is one and one half times that of the eye lens. 



In practice the ratio of the focal length of the field lens to the eye lens 

 may vary with the power, being 1 to 1.5 or 1 to 2 in low-power, and 

 nearer 1 to 3 in high-power eyepieces. 



Huygenian eyepieces as made for modern microscopes are not strictly 

 achromatic but have their focal length for blue light slightly longer than 

 for red light. In order to compensate effectively undercorrection, the 

 achromatic objectives may be designed to have their focal lengths for 

 blue fight slightly shorter than for the red light. 



