1 86 OPTICAL PROJECTION 



adapted) and require the most precise and delicate adjustment 

 of the object, for which they can afford ample time. The 

 second class consists of biological demonstrators ; who may 

 not only need equally precise adjustment, but also to pass 

 parts of a slide in regular succession over the screen, and 

 who likewise have no impatient general audience to grudge 

 the time such manipulation involves. 



The fitting or short tube answering to the body of the 

 compound microscope which carries the objective o, and 

 is screwed with the standard E. M. S. screw, is roughly 

 focussed by the rackwork E 2 , and finally adjusted for higher 

 powers by the fine-adjustment F, substantially made on the 

 pattern so well known in most * histological' stands. This 

 fitting slides into an outer tube, and it is advisable to have 

 several, so that different powers may be ready screwed in 

 place, and simply withdrawn or inserted. 



93. Amplifying the Image. It will often happen that 

 the direct image from the objective alone will not give the 

 best results. Suppose we want to exhibit an object double 

 the size it appears by a ^ lens. We may either double the 

 size of the image shown by that lens, or we may use a lens 

 of double power, \. But the results are different. By 

 amplifying the image we double the whole image, or nearly 

 so, as shown by the ^, and preserve what people roughly 

 understand by its ' depth of focus,' and the larger working 

 distance ; whilst the I lens will show a much smaller portion 

 of the slide, and the light cannot be proportionately condensed 

 into so much smaller a space, though it can be considerably 

 more so condensed. Hence, while really minute and sharp 

 sections or objects are usually shown better by a higher 

 power, more distinct objects of larger size are often best 

 shown with the amplifier, if more power is required. 



The image is 'amplified' for ordinary objects by inter- 

 posing a plano-concave lens A M between the objective and 

 the screen. This lens fits into the end of a tube, which 



