210 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Microscope is much larger, for it measures no less than 11 in. from the 
eye-cap to the objective front, and its stage area is 15*9 sq. in., viz. half 
as large again as that of the largest Continental instruments. The 
question may be asked, if a Microscope having a body and a stage of 
the above length and area were to be mounted on a horse-shoe stand 
of the Continental form, what size and weight of stand would be re- 
quired, so that both instruments might have the same amount of stability 
when inclined in any position? 
“ This Microscope originally had a circular hole in the stage, but 
in 1880 I had the brass cut away to the front, in order that the slide 
might be more easily tilted by the finger against the objective front, so 
that the working distance might be ascertained. By this means an oil- 
immersion 1/12 of any aperture can be quickly and safely brought into 
focus by means of the coarse adjustment alone. In 1882, Messrs. Swift 
and Son made a Microscope for me with a similar form of stage ; this 
is figured in the Journal R.M.S., 1883, p. 554, fig. 94. I have other 
Microscopes made on the same plan, and all give complete satisfaction. 
This form of stage is now largely adopted. 
“ The stage has a plain sliding bar, sprung ; this works so smoothly 
that it can be moved by the pressure of the finger acting only on one side. 
“ The second peculiarity is that the Microscope has no fine adjust- 
ment. Powell always insisted that the quality of the fine adjustment 
was one of the most important factors in determining the perfection of 
a Microscope. He also gave it out as an axiom that a Microscope, fitted 
only with a good coarse adjustment, was better than one which had, in 
addition to the coarse adjustment, a rickety fine adjustment. 
“ In this he was perfectly right. I can remember that my second 
Microscope had a worthless short lever- nose-piece fine adjustment; the 
result was that the fine adjustment was never used at all, focussing was 
performed solely by means of the coarse adjustment ; the fine adjust- 
ment was therefore only in the way, and contributed instability to the 
nose-piece, and it is perfectly clear that the instrument would have been 
far better without a fine adjustment. 
“ Powell was unwearied in his efforts to perfect the fine adjustment, 
and the perfection to which it has been brought in the modern Micro- 
scope is wholly due to him. As a matter of history, Powell’s fine adjust- 
ments were in advance of the requirements of the lenses of his day, 
which had very low optical indices ; in illustration of this, let me point 
out that a Microscope of his, with stage focussing, made in 1838, works 
a modern oil-immersion apochromatic of 1*43 N.A. with steadiness 
and precision. When the apochromatics were brought out by Prof. 
Abbe in 1886, the Continental fiue adjustments had all to be revised, 
because they were so defective that they were incapable of focussing 
■these new lenses with large optical indices ; while Powell’s, on the other 
hand, were quite equal to the emergency, although constructed upwards 
of thirty years previously. 
“ My own opinion is, and has for long been, that a direct acting screw 
fine adjustment (other than a differential),* however well constructed, 
* The differential screw fine adjustment wa3 first applied to a Microscope by 
Nobeit ; see Monthly Micr. Journ., 1869, p, 32-1. It was first suggested by Dr. Goring 
in 1830. 
