SOME SPECIAL PKOCESSES 265 



when dealing with transparent objects ; in fact, so much so is 

 this the case that if an object is, for instance, photographed at 

 100 diameters with an 8-mm. objective and is then photo- 

 graphed at 1000 diameters with a 3-mm. oil-immersion, using 

 a vertical illuminator in each case, the exposure, assuming 

 that all other conditions are the same, will be almost exactly 

 the same length of time in each case. The reason for this is 

 that the objective itself being the condenser, its working N.A. 

 increases exactly as its own N.A. increases ; the result being 

 that with the high-power objective the image of the source 

 of light is formed by a much wider beam and is of very much 

 smaller dimensions when projected on to the object. It 

 follows that the necessary exposure in each case would be 

 almost exactly the same, except that there is some slight 

 difference in the light-absorbing power of the objective itself 

 which must be allowed for. In actual practice it is found that 

 even with widely varying magnifications, when using the same 

 class of objective, the exposures differ but little. 



It does not come within the scope of this work to describe 

 the methods of dealing with metal surfaces to make them 

 suitable for microscopic observation. In general, however, 

 it may be stated that first the object requires to be ground or 

 cut so that it has a plain surface. This surface, then, has to be 

 polished so that any scratches or abrasions are obliterated. 

 The polished surface then requires either etching by suitable 

 solvents, or heating, and cooling quickly or slowly, so that 

 some differentiation, in cases where an alloy is being observed, 

 can be seen between the constituents. For further information 

 on this subject the reader is referred to the many books which 

 now deal with the micro-structure of metals, particularly the 

 works on ' Alloys,' by Mr. E. F. Law, and ' Metallography,' by 

 Dr. C. H. Desch. 



The object having been prepared, the first point in en- 

 deavouring to photograph it is to ensure that the polished 

 surface is at right angles to the optic axis of the microscope. 

 It will usually happen that metal preparations are irregular in 

 shape, and that only one surface and that the one under 

 observation is flat. This therefore must be set up exactly in 

 the position stated. The simplest arrangement for so doing 

 is to lay the specimen with its polished side downwards on to 



