THE METALLOGRAPHIC MICROSCOPE 



719 



The size of the aperture depends upon the size of the specimen, 

 but it must be large enough to permit the highest-power objective 

 used to come to its focal distance. Where a specimen is large, a 

 larger aperture has the advantage of exposing more of the surface 

 to view, facilitating the finding of such features as are visible mega- 

 scopically. Where the megascopic characters are of great im- 

 portance, the disk may be re- 

 placed by a ring, with three 

 converging arms attached 

 within it. These arms have 

 a profile like the cross section 

 of the disk. The ring and 

 arms must be ground to a true 

 lower surface, and the arms 

 must be polished to prevent 

 scratching of soft specimens. 



Instead of inserting the 

 post as shown in Fig. 1, it has 

 been found possible to use a 

 spring clip of the form shown 

 in Fig. 2, if the variation in 

 the thickness of the speci- 

 mens is not too great. The 

 ends E and E' hook over the 

 upper surface of the stage, 

 and support the spring, which 

 is made of thin spring brass, about one-half inch wide. If carefully 

 fitted the disk may be forced into place, where it will stay without 

 the necessity of threading. Using the second type of spring and 

 the fitted disk makes the attachment very inexpensive and at the 

 same time effective, especially if a number of springs of different 

 curvature for different sizes of specimens are made. 



It will be seen that, in addition to the fact that the necessity of 

 mounting the specimen is eliminated, the position of the stage when 

 in focus is the same without reference to the thickness of the speci- 

 men. This removes the necessity of stage focusing, and facilitates 

 the work when a comparative study of two or more specimens is being 

 made. Fig. 3 shows a microscope with the attachment as it is used. 



Fig. 3. — Photograph of microscope with 

 attachment in place. 



