600 IMAGE FORMATION, PROJECTION MICROSCOPE [Cn. XIV 



formation are taken up later in 858, in connection with 

 aperture. 



835. Amplifiers and oculars. When using the projection 

 microscope it is often desirable to magnify the screen image without 

 changing to another objective. This may be done with an ampli- 

 fier or an ocular. 



836. Image formation with an amplifier. The amplifier is 

 a negative lens or combination placed some distance beyond the 

 objective. Without the amplifier the objective would form an 

 image at /' (fig. 338). The effect of the amplifier (A) is to cause 

 rays to cross at / which would otherwise cross at /' and at the same 

 time the light from the objective O is rendered more divergent and 

 it covers a larger area on the screen than it would without the 

 amplifier (Fig. 126). 



When using the amplifier one must focus the objective slightly 

 farther from the specimen. 



837. Magnification due to the amplifier. The magnification 

 due to the amplifier is greater the shorter its focal length and the 

 farther it is from the objective. The same principle is employed 

 as with the telephoto-attachments to photographic objectives. It 

 has been found that an amplifier of -5 diopters (20 cm. focus) 1 1.3 

 cm. from the objective will give a magnification of 1.68 times and 

 an amplifier of -10 diopters (10 cm. focus) at the same distance will 

 magnify 2.5 times. (See also 356a for diopter, and for the 

 amplifier 3Q2a). 



838. Projection ocular. A projection ocular is required for 

 certain apochromatic objectives which are designed to be used only 

 with a compensation ocular, and when the microscope is used with 

 polarized light, otherwise it is not necessary to use an ocular, 

 although one may be used with any microscopic objective, see Ch. 

 IX and Ch. X under demonstrations and drawing with high powers 

 ( 401, 405, 477). 



The field lens Oi (fig. 339) in connection with the objective 

 forms an inverted image of the object at D. This image is in turn 

 projected by the eye lens or combination 0%, to the screen at /. 



