ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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•20 mm. the collar of such an objective needs merely to be set at 20 to give 
the proper correction, and consequently the best results. On the other 
hand, with an objective which is graduated on this system, the correct 
thickness of cover-glasses can be determined by obtaining the proper 
correction on preparations previously made, but on which the thick- 
ness of cover-glass is not noted, and the thickness may be marked on 
them for future convenience. To do this successfully, however, con- 
siderable experience is requisite. All the other scales give the correct 
tube-length in inches and millimetres for covers corresponding to them, 
and in this manner offer a ready and definite means of correction. The 
tube-lengths required for the thinnest and thickest covers are so extreme 
that probably no convenient means for obtaining them can be practically 
arranged, but they can be so approximately if not entirely. At any 
rate, the micrometer will detect the requirements before using the 
covers, and those deviating considerably from the normal can be used 
on objects for use with low powers only, in which case the effect will 
not be very appreciable. 
In this system I do not overlook the fact that variation in tube- 
length involves a variation in magnifying power, but except in cases 
when micrometers are used, I consider this of secondary importance, as 
it always is in comparison with the results obtained in resolving and 
defining power. 
This system involves four conditions : — 
First. That all cover-glass be measured before using, and that the 
thickness be noted on the preparation. 
Second. That for convenience all draw-tubes be marked in inches 
or millimetres, or both. 
Third. That adjustable objectives be corrected according to this 
scale. 
Fourth. That the same tube-length and cover-glass thickness be used 
in all original corrections of objectives. 
As regards the first condition, there are many microscopists now 
who measure all their covers before using them, but the mere knowledge 
of thickness has been of no value up to the present time, because this in 
itself has been no guide in obtaining better results except by approxi- 
mation. My aim in connection with this system has been to devise an 
instrument which shall possess a high degree of accuracy, and shall still 
be so inexpensive that its price should be no obstacle to its general use. 
The celebrated preparer of objects, Mr. J. D. Moller, and others, 
have kindly agreed to mark the thickness of covers on their objects, so 
as to aid in the introduction of this system, and other preparers can no 
doubt be induced to do so if its advantages can be proved. 
As regards the second point, many manufacturers now graduate 
their tubes, and modern requirements demand that this should be more 
generally done. Our company intends, as soon as it can possibly 
arrange to do so, to graduate the tubes of all its instruments. 
As to the third and fourth conditions, I cannot, of course, presume 
to ask manufacturers to adapt their standards to this system. While it 
will be a convenience to a large number of microscopists, I must leave 
it to the merits that this system may possess, to exert their influence in 
this direction. 
1891. 
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