158 PEACTICAL PHOTO-MICKOGKAPHY 



position B ; in fact, at B there will be nothing but a large and 

 diffused patch of light. The images observed at the points A 

 and B in this case will therefore be totally dissimilar. This 

 is the method actually adopted in correcting for a desired 

 tube-length or thickness of cover-glass, and opticians when 

 effecting this correction in the course of manufacture usually 

 have some object with which they are familiar and of which 

 they know the appearance under varying conditions. On 

 focussing such an image first above and then below the focus, 

 and successively performing this operation and comparing the 

 two appearances, it can be determined whether the objective 

 is properly corrected for the conditions under which it is to be 

 used. It must at once be said that it is not by any means 

 easy for a novice to appreciate the differences, and to get 

 correct adjustment ; a good deal of practice is necessary, and 

 in fact only the trained microscopist can obtain the best 

 correction in actual practice. When working with an object 

 which has a cover-glass of unknown thickness it will be found 

 best to focus and to effect the correction by means of some 

 small dust-particle in the object. On rapidly focussing this, 

 both above and below its proper focal point, the difference in 

 the images will become apparent. The objective must then 

 be corrected by its correction- collar, if it has one, or by altering 

 the tube-length of the microscope until its image is such that 

 it appears exactly the same whether the focussing is above or 

 below the proper focal point. Alternatively, any fine line or 

 hair in the object may be utilised to perform the operation. If 

 the lens is not properly adjusted, the appearances on opposite 

 sides of the true focus will not be identical. 



The best object for demonstrating the effect is a Podura 

 (Lepidocyrtus curvicollis) scale. This is the one most uni- 

 versally employed by opticians to correct their objectives 

 both for thickness of cover-glass and for length of tube. Fig. 57 

 shows at a the appearance of a portion of the scale that may 

 be expected if it is in correct focus and the lens perfectly 

 adjusted; at fc, the appearances well outside both sides of 

 the focus when the adjustments are correct, and at c the 

 appearance as it is just going out of focus on either side ; d and 

 e show the two distinct appearances, one on either side of the 

 focus, when the objective is not in adjustment ; / shows the 



