FUNCTION AND PARTS OF THE MICROSCOPE, 21 
microscope, commonly two eyepieces and two or more 
objectives of different magnifying powers. The lenses of 
some makers are designated by arbitrary symbols; as a 
rule, however, they are marked with numerals which 
indicate what is known as their equivalent focus, or the 
focal length of a simple converging lens which would 
In. / 
Obj. 
Fic. 16.—RELATION oF DisTaNCE FROM PRINCIPAL Focus To Size 
AND PosiTION oF IMAGE. 
produce an image of the same size as that formed by 
the lens in question. The larger the number the lower 
will be the power. Thus a 14-in. eyepiece magnifies 
less than a 1-in. eyepiece, and a $-in. objective less than 
a #in. objective. The commonest high-power objec- 
tive is the 7s, and the 7; is the highest objective which 
can be practically constructed. By making use of ultra- 
violet rays with a wave-length only half that of ordinary 
‘light the Zeiss Optical Company has recently been able 
to construct instruments of much higher power. The 
lenses in this case are of fused quartz, and the image 
produced, being of course invisible, is recorded on the 
photographic plate. 
