110 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
At the meeting of this Society in 1887 at Pittsburgh, he gave the 
results of his efforts, which show some astonishing facts. I would here 
say that while for a long time I had felt that a system that would 
permit the full utilization of the optical capacity of objectives of different 
makers under varying conditions of cover-glasses was desirable, I was 
then forcibly impressed with the absolute necessity of a plan which 
would offer this advantage. One is as much surprised by the differences 
in cover-glasses used by various makers in correcting non-adjustable 
objectives, as by the great differences in the length of tube, which in- 
fluences so considerably the spherical aberration of the objectives. 
With a thickness of 0*1 mm. for the thinnest, and 0*25 mm. for the 
thickest, it is only too apparent that with the additional variation in 
lengths of tubes, it is bevond the power of the microscopist to obtain 
even approximately the best results from his objectives. More than 
this, a large quota of the advance made in recent years in the capacity 
of objectives has been lost. 
As Prof. Gage states, “ A uniform thickness for cover-glass for 
unadjustable objectives seems also desirable,” and this would be the 
easiest solution of the question, but while, on the one hand, the makers 
of objectives have not yet agreed to use one standard on account of the 
technical difficulties involved in departing from their established 
precedent, on the other, the microscopist would hardly be willing to 
bear the expense which would be occasioned by the loss of cover-glass 
not conforming to the standard, in order to use those of one thickness. 
This expense might be greatly reduced by using selected covers of one 
standard on objects for all medium and high-power objectives, and the 
balance on all other preparations, on which only low powers would be 
used, but this would of course be of little avail in face of the fact that 
manufacturers follow no standard. 
The greatest difficulty is met with non-adjustable objectives. As is 
well known, compensation for thickness may be obtaiued in the proper 
adjustment of tube-length, but while not all Microscopes are suitably 
provided with draw-tubes, the requisite experience and skill are lacking 
with a large number of microscopists to make the correction properly 
in this manner, as well as in objectives especially provided with collar 
correction. I am sure that microcopists of long experience will bear me 
out in the statement that results with adjustable objectives depend upon 
individual skill, and that many such objectives now in use fail to give 
results corresponding to their capacity. It would seem, therefore, that 
any system to permit the full utilization of the capacity of objectives 
should depend on no personal factor, in fact, should be mechanical, and 
this I have followed out in the system that I shall explain. 
In an objective corrected for normal thickness of cover-glass there 
will be spherical over-correction with thick covers and under-correction 
with thin covers, the amount of correction varying in a different ratio to 
the amount of variation from the normal thickness. The chromatic 
correction will also lose correspondingly, but not to so high a degree. 
While a deviation of a few hundredths of millimeter in either direction 
will not signify, that which occurs in covers classified in price lists under 
one number is sufficient seriously to affect, and in the higher powers totally 
obliterate the definition, which under normal conditions it may possess. 
