1924 



Journal of Applied Microscopy 



section so that the etched surface is parallel to the ground glass base, a 

 microtome is a great convenience (M Fig. 8). Projecting from a cylindrical 

 metal base three inches in diameter, is a threaded upright three inches 

 high and one and one-half inch in diameter. A cylindrical nut or collar 

 three inches high and two and one-half inches outside diameter screws on 

 the threaded upright. A small circle of chamois skin is placed on the top of 

 the threaded upright to protect the etched face of the micro-section. To mount 

 a section place it face down on the chamois skin, press upon the upper project- 

 ing portion the lump of beeswax and upon this place the ground glass (ground 

 surface down). A few revolutions of the collar will cause the glass to rest upon 

 the upper edge of the collar and the adhesion of the glass and beeswax to the 

 specimen may be made complete by slowly turning the collar down with one 



hand while keeping the glass 

 base in close contact with the 

 collar-top with the other hand. 

 In this manner, no matter how 

 irregular the section, the paral- 

 lelism of the etched surface 

 and the glass base may very 

 quickly and accurately be ob- 

 tained. 



The magnification used in 

 ordinary investigations ranges 

 from 3(1 to 10(1 diameters. 

 The micrographs accompany- 

 ing this article are uniformly 

 (j5 diameters. Photo-micro- 

 graphs 4 and 5 are from the 

 same bar of medium carbon 

 steel etched with tincture of 

 iodine for ten seconds. The 

 difference in the size of the 

 grains of the two specimens is caused by the degree of heat to which the two 

 sections were subjected ; 4, having been heated to 1200 degrees Centigrade 

 and 5 to 630°C. 



When the examination shows that the specimen has been properly prepared 

 the observer selects a spot representative of the micro-section under considera- 

 tion. Here the temptation arises to take that which is unusual rather than that 

 which is characteristic or to photograph " freaks " instead of normal structures. 

 Unusual structures frequently occur in metals or alloys that have been subjected 

 to a variety of treatments in the way of reheating or reforging. Figs. 6 and 7 

 were taken from the same micro-section of steel within a circle of less than 

 one-half inch radius, and show a variety sometimes found in a single specimen. 

 Arranged upon a well-equipped optical bench, in the following order, are 

 these parts (see Fig. S) : a source of light, the best (after sunlight) being an 

 automatic-feed arc lamp (A), surrounded by a suitable case, with an opening at 



Fig. 



