HENRY M. SAYERS 169 



much magnified to fill the field of view, so that the effective brilliancy 

 is reduced. Moreover the separate turns of the spiral 

 become visible in a critical image. These small lamps 

 can be run from a few ignition cells, so that they are 

 convenient for portable use. Run from an ordinary- 

 supply circuit they require either a resistance or a transformer to 

 reduce the pressure to 6 or 8 volts. Good work has been done at high 

 powers on steel with a 6-volt 4-ampere half-watt lamp, but the 

 exposure is two to three times as great as with a Nernst lamp. Up to 150 

 diameters this exposure is reckoned in seconds, so the difference is 

 not important, but for high powers the exposure goes up to several 

 minutes (15 to 20), it counts where much work has to be done. Long 

 exposures are objectionable not only because they limit the speed of 

 work; but also because they increase the risk of disturbance of the 

 image by vibration. 



The illuminations needed to give short exposures are far too bright 

 for comfort in visual examination. The interposition of a piece of 

 fine-grained ground glass is a simple remedy. It can be put anywhere 

 betw^een the light source and the vertical reflector and no adjustment 

 is disturbed. The final focussing must be done on the ground glass of 

 the camera, whore the full illumination will not be found excessive for 

 the purpose. 



Anyone starting on micro-naetallography will find his initiation 

 much easier if he tries first visual and photographic work by trans- 

 mitted light on transparent objects. As the sub-stage condenser is 

 independent of the objective it is much easier to try variations of focus 

 and illumination, and the knov/ledge so gained helps very much to 

 recognise proper and improper condition in opaque v/ork. Good objects 

 for such training are section of Echinus spine for low powers, and 

 diatoms of various fineness of structure for the higher ones, A student 

 who can get a good dark round negative of Echinus spine at 100- 

 diameters, and good " black-dot " negatives of Pleurosigma Angulatum 

 or Surirella Gemma at 1,000 diameters with an oil immersion objective 

 will find work on metals much simpler than if he came to it without 

 such practice. The superior resolving power and definition of a given 

 objective with the sharp image of the illuminant focussed on the 

 object from a sub-stage condenser of aperture comparable to that of 

 the objective will be appreciated. 



In metallography (excepting with the very low powers) the objec- 

 tive plays the part of the sub-stage condenser as well as its own. Tc^ 

 obtain a sharp image of the source of light upon the object when the 

 object is focussed to the eye-piece, certain distance relations betrCveen 

 the illuminant, the objective, the object, and the image plane must be 

 observed. They are simple. The light source must be at the same 

 distance from the back lens of the objective as the image plane, the 

 distances being measured along the path of the light in each case. 

 Obviously the source and its image on the object are at the conjugate 

 focii of the objective; and the object and its image are at equal conjugate 

 focii. An immediate consequence of this relation is that the illumi- 

 nated field or useful part of the image formed by the objective is of 

 the same dimensions as the source (real or virtual) of light itself. If, 

 for example, the source of light is the crater of an arc, one-tenth of an 

 inch in diameter, the usefully illuminated part of the real image 



