160 PEACTICAL PHOTO-MICEOGKAPHY 



images, the position of the image either above or below a 

 particular focal point is quite easy to observe. It will be of 

 educational value to watch the effect on other parts of the image, 

 varying the two conditions referred to, as it will facilitate these 

 adjustments being made on an object that does not lend itself 

 to this particular treatment. 



It should be noted that in all cases where such corrections 

 are to be made the highest power eye-piece that the objective 

 will stand should be used. In the case of achromats this 

 will therefore be an eye-piece that does not enlarge the image 

 more than ten times, while with the apochromatic objectives 

 an ocular magnifying eighteen times is the most convenient 

 one to employ. 



It is far preferable, where possible, to ensure that the object 

 is covered by a cover-glass of the correct thickness, and to use 

 the lens at the correct tube-length as indicated by the maker. 

 The method of correcting for thickness of cover-glass by 

 alteration of the tube-length is not so satisfactory ; in fact, 

 when using medium or low powers, some variation from the 

 correct tube-length is not so serious under ordinary condi- 

 tions of work. It has been found by experiment that an 

 alteration of one-thousandth of an inch in the thickness of the 

 cover-glass when using a particular high-power dry lens is 

 equivalent to an alteration of seventeen inches in the length 

 of the draw-tube. It is not advisable to infer from this that 

 the length of the tube has a negligible influence on the correc- 

 tion, although its effect is in general somewhat exaggerated. 

 Its importance becomes much more evident when lenses over 

 one-sixth of an inch are used. The correct length of tube 

 may be determined by observing the image in the same manner 

 as that already described for correcting for thickness of 

 cover-glass. The appearance of the image is exactly the 

 same when there is any substantial variation from the correct 

 tube-length as in the case where the correct thickness of 

 cover-glass is not being used. 



The Use and Adjustment of the Sub-stage Condenser.- 

 When using objectives of two-thirds or a half inch, as well 

 as higher powers, a sub-stage condenser must be employed. 

 This should preferably be of the achromatic variety, and 

 should be carried in a centreing sub-stage together with an 



