THE MICROSCOPE. VA: 
optical way as the adjustable forms, because they have no means 
of correction for certain changes in the image produced by cov- 
ering the object to be examined by the thin glass always employed 
for the latter purpose, and because they must always be used as 
the maker decided to be best, whereas many contingencies arise 
that he has not provided for, when the adjustment becomes nec- 
essary, if the microscopist desires a fine image. When he makes 
the non-adjustable objectives the optician fastens the component 
lenses in the position that gives the best image with a certain 
length of the body-tube, but what that length was we often have 
no means of knowing, although we may be certain that it was 
nearer ten inches, the standard length, than the five or six of the 
short tubes. But if the non-adjustable objectives are perma- 
nently adjusted by the optician for a certain length of body, 
would even the beginner in the use of the microscope suppose 
that such objectives would give as good an image when 
used on a tube of any other length? If he should imagine 
that he would be mistaken. No two persons can use the 
opera-glass or spy-glass in succession without changing the 
focus, or the relation of the lenses to each other, because 
no two persons have precisely the same excellence of eye-sight, 
or the same focus to their eyes. And it also happens that non- 
adjustable objectives made to be used on a ten inch body will 
fail in some important particulars when used on a shorter tube ; 
and one prepared for a short tube will fail in some of its fine 
points when employed on a long body. In adjustable objectives 
these difficulties may be overcome by separating or approximat- 
ing the component lenses by turning the collar, that is, the ob- 
jective may be corrected, but in the non-adjustable objectives the 
microscopist who has trained his eye for perfect images of the 
object examined, will be forced to take what he may get, with 
no power to better the result. For this reason the short body- 
tubes are objectionable. Zeiss, in his English circular, refering 
to his apochromatic objectives, says: 
“The objectives are made either for a tube length of 160 mm., 
or of 250 mm., with the exception of the three of 6 mm., 12 mm. 
and 24 mm. focus, which are exclusively for the tube of 250 
mm., these lenses not being so favorable for the shorter tube. It 
is important that the length of tube be kept in consideration, 
because any change in same will considerably diminish the per- 
