INTKODUCTOKY 3 



was only practised by the few, although it is unquestionably 

 the fact that the photographs then produced by two or three 

 well-known workers have seldom been equalled, and in their 

 class have never been excelled. With the advent of the * dry- 

 plate ' method all this was changed, and photo-micrography 

 began to take up the position in microscopy that its earliest 

 devotees expected and hoped for it. 



The difficulty already mentioned, of delineating photo- 

 graphically an object, the details of which extend through a 

 series of planes, is certainly a considerable one, and in the 

 majority of cases, almost impossible of solution. It is, however, 

 quite within the range of possibility to select the planes that 

 give the best general appearance, and, by careful methods, 

 which will hereafter be described, to select the focus which is 

 most suitable for a particular object, and to exclude more or 

 less that part of it which is useless. It is, however, a fact that 

 in microscopical work the objects to which this objection applies 

 are comparatively small in number; and more particularly 

 with the present methods of section-cutting, and of staining 

 of preparations where the sections themselves are thin, the 

 difficulty will not appear at all. 



Photo-micrography now embraces a very wide field of 

 work. It is applied particularly in pathology, bacteriology, 

 and histology ; but in later years its application to metallurgy 

 and petrology has opened up a still larger field. 



Its advantage over any method of drawing must be obvious, 

 as it is impossible to get an appearance on the photographic 

 plate, assuming of course that the work is carried out honestly 

 and with due regard to the principles involved, other than 

 that which actually appears in the object itself. By its use 

 too, the factor known as ' the personal equation ' is eliminated, 

 so that in recording the results of research only a truthful 

 rendering of objects is possible ; or at least no definite 

 variation to suit the end in view can be made, as is possible 

 when drawings are employed. It is also obvious that many 

 objects, either from their intricate structure or fineness of 

 detail, cannot be successfully drawn : but by the photographic 

 method no detail is too fine to be delineated with perfect 

 accuracy. 



It should at once be said that to acquire any considerable 



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