150 METALLURGICAL PHOTOMICROSCOPE : 



With the author's camera and microscope, magnifications ranging 

 from 9 to 23 can be obtained, but with a longer camera it would 

 be quite possible to reach 30 or 40 diameters. As mentioned above, 

 the tube of the author's microscope is very wide and comparatively 

 short (2 in. diameter and 4| in. long), and this enables one to 

 use practically the full field of the 35 mm. lens; with a narrower 

 tube, of course, part of the field would be cut off. In such cases 

 one may do without the microscope and mount the lens on a small 

 fitting (with either spiral or rackwork focussing) on the front of 

 the camera. In one or two cases the author has done this, and 

 has succeeded in obtaining a 6 J in. circle at 9 diameters (i.e., a field 

 of .72 in.). The definition in this case was not quite perfect round 

 the edge, but it was sufficiently good for the purpose. 



It may not be out of place to mention that when taking a 

 photomicrograph without using an eye-piece it is necessary to avoid 

 reflection from any metallic surface inside the microscope tube. 

 The latter should be coated with a dull black varnish, but it is 

 generally advisable to put in a lining of black cloth. Such a lining 



^sr 



Fig. 12. 

 Illuminating- system for lowest powers (up to about X5). 



can be easily made by gumming a piece of black cloth of the required 

 size on to a similar piece of fairly stiff paper, then rolling into a 

 cylinder (cloth inside), which can be slid into the microscope tube. 



For still lower powers one may use a photographic lens of 3 in. 

 to 5 in. focus; the one used by the author is a 5^ in. Holostigmat 

 by Watson, and with this he can reproduce from natural size up 

 to about 4 diameters. In this case a different system of illumination 

 is employed. For the 45° reflector a thin lantern slide cover glass 

 is used. (It is possible to obtain thin microscopic cover glasses 

 in sizes up to 4J in.x3:J in., but such glasses are very fragile.) A 

 large piece of ground glass is mounted close to the section as shown 

 at A in Fig. 12, and the condenser B throws a parallel or slightly 

 divergent beam of light on this. The idea is to produce an evenly 

 illuminated disc of light on the ground glass, and the light from this 

 is reflected on to the section by the 45° reflector C. Such low 

 magnifications are es])ecially valuable with sections etched with one 

 of the " copper " reagents (such as Stead's, Rosenhain's, or Le 

 Chatelier's), which require a low magnification as a general rule. 

 Fig. 13 is an example of this — it represents a section (x4 diameters) 

 from a small sample ingot (taken for analytical purposes from an 



