156 PEACTICAL PHOTO-MICKOGEAPHY 



for this variation in thickness when the object is mounted in a 

 medium of approximately the same refractive index as glass, for 

 under these conditions no refraction of the light occurs at the 

 surface of the cover -glass. When work of the most critical 

 nature is being carried out, particularly when the resolution of 

 the structure of diatoms mounted in media of high refractive 

 index is attempted, this does not apply. The correction 

 necessary in these cases, however, is usually very small and such 

 that it may be effected by alteration of tube-length; in this 

 class of object great care is exercised and considerable experience 

 is necessary in making a preparation, and it is usual in such cases 

 to calibrate carefully the cover-glass before mounting up. 

 It is partly for this reason that the unskilled microscopist so 

 generally prefers an oil-immersion objective, for it is not so 

 sensitive to any small variation from the required conditions 

 as a dry lens. The advantage of the dry lens is that it is of 

 greater focal length, and if the magnification required can be 

 obtained with it, then the additional working distance is often a 

 great advantage. With some objects where a thick cover-glass 

 has been used a dry lens is the only possible one to adopt, and 

 in such cases the necessary correction must be made by the 

 correction- collar, or the tube-length must be shortened. 



The adjustment by observation of a lens with a correction- 

 collar is not a very easy matter except to a microscopist of 

 considerable skill and experience. It is obviously impossible 

 to measure accurately the thickness of a cover-glass on a 

 mounted object ; but where a microscope possesses a graduated 

 milled head to its fine-adjustment, an approximate estimate of 

 its thickness may be obtained without much trouble. If the 

 objective in use also has marked on it the correct position of 

 the index on the correction- collar for a given thickness, as 

 it should have, it is then only necessary to set the collar for 

 the determined thickness. If the specimen should be an 

 ordinary bacteriological cover-glass preparation, all that is 

 required is to focus a bacterium in the preparation, and then to 

 raise the objective by the fine-adjustment until some very small 

 particle of dust on the top surface of the cover-glass comes into 

 view. The difference of the focus, as shown by the graduation 

 of the milled head, will then indicate the thickness of cover-glass 

 in use. To ascertain the thickness of the glass, however, the 



