MEDIUM-POWEE PHOTO -MICROGRAPHY 117 



diameters on the ground glass held at lo inches from the eye-piece ; but if now 

 it be removed to 20, inasmuch as 20 divided by 10 gives a quotient of 2 our 63 

 diameters' magnification would become 126, Similarly, if we drew out our camera 

 30 inches, as 30 divided by 10 gives a quotient of 3, our object would now be 

 magnified 189 diameters. It is seen, therefore, to be a very simple matter to judge 

 the amount of magnification an object has received when using the perfected system 

 of the apochromatic series of lenses. This, be it understood, is not due to apo- 

 chromatism considered as such, but because all objectives are really of the focus 

 stated. But when employing achromatics, which are not so perfect in their focal 

 lengths, both as respects the objective as well as the eye-piece, another method has 

 to be employed. 



Place any power— say an achromatic one inch, which very likely is really a two- 

 thirds inch — on the microscope, and use first what is called an " A " eye-piece. 

 Having procured a stage "micrometer" (which is only another name for a cover- 

 glass ruled with lines by a diamond a definite distance apart and fixed on an 

 ordinary 3 x i slip), and having focussed any two of the lines which are separated 

 by an interval thought to be of convenient size, the ground glass is placed at a 

 distance of 10 inches from the eye-piece. We then take a pair of compasses and 

 measure directly the interval of these lines apart, and divide it by the distance 

 which we know exists between the two lines on the micrometer. It is very obvious 

 that the quotient thus obtained is the linear magnification produced by the lenses 

 —the objective and the eye piece— as they then stand. It is the same, in point 

 of fact, as would be seen by the normal eye looking down the instrument through 

 the eye-piece save that it is reversed in position. 



This process is repeated with each eye-piece, and with each objective in succes- 

 sion. It is now very evident that we can apply the figures thus obtained, whenever 

 we use a similar combination of objective and eye-piece, always recollecting that the 

 figures only apply to the camera length of ten inches. If this be increased the 

 magnification is augmented in just the same way, and just the same manner as 

 already explained when employing apochromatics. The photographer should 

 do these computations for his photographically corrected achromatic objectives and 

 his eye-pieces with the greatest of care, as when once done— provided the 

 tube-length of the microscope is kept the same, and the camera length does not 

 exceed ten inches— the results will be always correct.^ 



* The direct measurement of minute quantities such as the number of lines to the inch in exceedingly small diatoms, 

 like those in amphipleura pellucida, is not given here as it was thought to be out of place, but for the assistance of tho5e 

 interested in the subject, it is fully described in the Appendix. 



