GENEEAL PKELIMINAKY PEEPAEATIONS 198 



photograph therefore not being taken through the lens itself. 

 The use of the lens is simply an arrangement to enable the 

 eye to see the plane of the preparation that is projected on the 

 ground-glass. The authors of this method recommend that 

 trial photographs should be taken with an object such as 

 a stage micrometer, and that this should be photographed 

 with different combinations of lenses and length of camera, 

 so that the lens to use for a given magnification may be 

 determined. 



Obviously this method has its limits, since it only enables 

 the worker to obtain certain magnifications for which he has 

 a focussing-lens. Further, the accommodation of the eye, which 

 in some cases is very considerable, is not allowed for ; hence 

 it often happens that an object which seemed visually to be in 

 focus, is only so in consequence of some strain on the eye, and 

 when the image is recorded on the photographic plate the 

 result is by no means sharp. It would appear, therefore, 

 that for general work, where widely different magnifications 

 are required and preparations of varying character have 

 to be photographed, the only practicable method is that 

 originally described, viz. to focus the image in the plane that 

 will ultimately be occupied by the photographic plate. 



Magnification. The magnification of microscopic objects 

 is usually expressed in diameters, and not in terms describing 

 superficial area. An object is therefore said to be magnified 

 a certain number of ' times ' this referring simply to the 

 number of diameters that it is enlarged. The same general 

 conditions apply as to any enlarged image projected by an 

 ordinary lens such as a photographic or a projection one. 

 The magnification therefore of a projected image if the 

 superficial area is considered varies to exactly the same degree 

 as the intensity of illumination, i.e. as the square of the 

 distance. In visual work magnification is estimated at a 

 distance of ten inches from the eye-lens of the eye-piece, as 

 this is the proper distance for normal vision. As a rule, 

 achromatic objectives are described as of somewhat longer 

 focus than they actually are, so that a one-inch objective is 

 often something less than that, and may be as short as three- 

 quarters of an inch. An objective of exactly one inch focal 

 length would magnify ten times at a distance of ten inches 



