116 



PHOTO-MICROGRAPHY 



superficial valuation, such as so many "areas " or so many " times," Objectives, then, 

 as well as eye-pieces, are said to magnify in terms of diameters and not in " areas," 



We now pass on to describe how to ascertain the amount of diameters an object 

 is magnified when seen in the microscope or when photographed on the plate. 



If we use apochromats this is a very simple matter, for each objective is of an 

 accurate focus, and so the initial magnifving power is easily known, and so, too, 

 with each compensating eye-piece. Here follows a list of the amounts universally 

 adopted for the standard magnifying power of objectives of different foci. It will 

 be convenient for future reference ; — 



A lens of i inch or 24 mm. focus magnifies io'5 diameters. . 



15-5 



2I-0 



42-0 „ 

 63'o » 



125-0 „ 



What this really means is that if any lens of exactly an inch focus is placed 

 on the microscope and a piece of ground glass held at a distance from it of 10 inches, 

 the magnification of the object is 10*5 diameters. 



Now to calculate the final amount of the amplification of the image as 

 seen in the microscope wlien the eye-j)iece is added, it is only necessary 

 to multiply the initial power of the lens by the initial power engraved on 

 the eye-piece, and the result is the total amount of the magnification as seen 

 by the normal eye when looking through the instrument. This, then, is the 

 simple rule for visual purposes when using apochromats. But when we come 

 to photo-micrography, where the image is projected on to the plate or the ground 

 glass, another factor comes into play — it is the length of the camera extension 

 employed. If the ground glass be placed at ten inches from the eye-piece it 

 will be found, although the object is seen reversed, the resulting magnification 

 is exactly similar to what is visually seen on looking down the instrument, and so 

 equals the initial magnification of the objective multiplied by that of the eye- 

 piece. But when the length of the camera extension is increased the amplification 

 is then directly in accordance with the amount of the camera stretch beyond this 

 ten inches. For example, if we place an object on the stage and use the inch 

 objective with a six eye-piece we should find the object would be magnified 63 



1 



8 

 1 



1 2 



16 



