170 ILLUMINATION IN MICRO-METALLOGRAPHY: 



formed by the objective will also be one-tenth of an inch in diameter. 

 This identity of dimensions is independent of the power of the objective. 

 A two-third inch with a power of 8 and a one-twelfth inch wath a power 

 of 100 with both give an illuminated circle of one-tenth inch diameter 

 in the image plane, provided that the conditions of critical illumination 

 are observed. If the total magnification on the camera screen is ten 

 times the objective image magnification, the effective field will be one 

 inch in diameter. This is generally too small for practical use, and 

 is much smaller than the field which the objective can cover. One 

 wants a field at least three inches in diameter to cover a quarter plate. 

 The assumed magnification of ten due to the ocular and camera length 

 combined is about as much as the best objectives will usefully stand. 

 So that to cover a quarter plate the radiant should be from a third to 

 a half inch in diameter. 



Unfortunately the available sources of light are of small area. An 

 arc crater of a quarter-of-an-inch diameter corresponds to an arc cur- 

 rent of 30 to 40 amperes, and is not really large enough. As the 

 crater diameter increases only as the square root of the current, one 

 would require a searchlight arc, with many tens of amperes to give a 

 crater three-quarters of an inch in diameter, which is about the ideal 

 size to fill an eye-piece. Such an arc is not practicable. Even a- 40 

 ampere arc gives out too much radiant heat to be brought within 

 the few inches of the microscope corresponding to the posterior focus 

 of the objective. The same difficulty of small area is true of the 

 ether available sources. The 100 candle-power Pointoliteihas a radiant 

 surface about one-tenth of an inch diameter. The Nernst and half- 

 watt lamps have filaments of much smaller diameter. There seems 

 no good reason why a half-watt spiral lamp filament should not be made 

 one-third or one half-inch diameter. There may be manufacturing 

 difficulties, or it may be that the makers have not seen that there is 

 any use for such lamps. If made the spiral, or rather helix should be 

 flattened to bring the radiant surface as nearly as possible into a plane. 



The actual radiant surface therefore has to be magnified in some 

 way to give a field of sufficient area. The simplest way is to use a 

 short focus condenser to project near the upper lens of the objective 

 an image of the radiant. This can be focussed on to the stop of the 

 vertical illuminator, and the fine focussing done by eye. The image 

 thrown on the object is that of the aperture of the condensing lens 

 which is then at the posterior focus of the objective. A condenser of 

 the Nelson type of two inches full diameter, stopped down to one 

 inch aperture works well. It must be carefully centered toithe radiant, 

 and both must lie on a Hne at right angles to the optical axis of the 

 microscope. To make these adjustments readily, some form of 

 mounting equivalent to an optical bench, with vertical and horizontal 

 movements to either the lamp or the lens is necessary. If the lens 

 is always used for the same stand, it can be fixed at the height of the 

 optical axis, and the adjustments for centering made on the lamj) car- 

 rier. Movement to and from the microscope to adjust the lens dis- 

 tance to the optical tube length in use — which may be different with 

 diff^e'rent combinations of objectives and oculars — and some movement 

 parallel to the body to allow for the range of movement of the illumi- 

 nator aperture, are necessary. These statements hold true for any of 

 the auxiliary arrangements described. 



