1905-1906.] Address by the President. 333 



and the combination is called an eyepiece. The function of 

 the field-glass is to gather together the rays proceeding from 

 the object-glass and bring them to a focus at the diaphragm, 

 where the real image of the object is formed ; and the func- 

 tion of the eye-glass is to form an enlarged virtual image of 

 this real image. It will thus be seen that there are two 

 enlargements of the object viewed — one by the object-glass 

 and the other by the eyepiece ; and it is these two multiplied 

 together which gives the final enlargement. There has been 

 a special form of eyepiece constructed, called a compensating 

 eyepiece, for use with apochromatic object-glasses. Its purpose 

 is to correct certain residual errors which could not be cor- 

 rected in the object-glass. 



The object-glass and eyepiece, togetlier with a mirror used 

 for reflecting the light upon the object, are called the oiotical 

 part of a microscope. They are bound together by the 

 mechanical part, which consists of several pieces, called col- 

 lectively the stand. The essential part of the stand is the 

 tube into the lower end of which the object-glass screws, 

 while the eyepiece slides into the upper end. There are two 

 conventional lengths of tube — the English form of 250 mm., 

 and the Continental form of 160 mm. This is called the 

 mechanical tube-length. There is another length of tube 

 called the optical tube-length — that is, the distance at which 

 ,the object-glass forms the real image of the object. Of course, 

 whatever length of tube is used, the object-glass has to be cor- 

 rected accordingly. Each length has its advantages. A con- 

 venient form is to have two or more tubes sliding inside one 

 another, so that the mechanical tube-length can be lengthened 

 or shortened to suit the object-glass in use. 



The tube is supported upon a foot, and the little table 

 attached to the foot for the support of the object under 

 examination is called the stage, in which there is an opening 

 for the admission of light reflected from the mirror placed be- 

 neath. The tube must be provided with some means of vertical 

 motion, so that it can be approached nearer to or withdrawn 

 from the object, according as the object-glass is of shorter or 

 longer focus. In the better class of instruments there are two 

 means of mechanical motion — one giving a quick movement 

 and the other a slow movement. There is perhaps no part of 



