34 
NOTES AND MEMORANDA. 
of my microscopes, while others fit loosely the nose-piece of my other 
instrument, although both microscopes have been supplied by the 
makers with the so-called " universal screw " ! Moreover, I have 
seen modern object-glasses (manufactured since the introduction of 
the universal screw), by one of the leading opticians, having different 
gauges of universal screw, and by another not only object-glasses, but 
adapted for analyzers, Brooke's nose-pieces, &c. When using high 
powers with a microscope having a concentric rotating stage (which 
is now considered almost a necessary addition), these variations of gauge 
render the stage eccentric, and no doubt very often the rotation of a 
stage is condemned, and the workmanship considered imperfect, when 
the fault lies in the inaccuracy of the so-called universal screw of 
either the object-glass or of the microscope's nose-piece, and fre- 
quently of both. I am quite aware that the smallest particle of dust 
in the object-glass screw will cause eccentricity, but this drawback is 
not a permanent one ; it is bad enough to have it when it occurs — 
there is no necessity to make eccentricity both a feature and a fixture ! 
With a universal screw, if we could not get in every instance perfect 
concentricity when rotating the stage, we should certainly approach 
it much nearer than we do now ; of course, accurate workmanship 
being always taken for granted. Besides the above inconveniences, 
there is another — the great difficulty and trouble in centring achro- 
matic condensers of large angle of aperture with high powers, by 
different makers, having different universal screws. The ' Royal 
Microscopical Society have undoubtedly conferred a great boon upon 
microscopists by introducing the present "universal screw"; but 
could not an effort be made to render the screw really universal by 
causing the Royal Microscopical Society's gauge to be adopted by all 
the London opticians? Some technical and practical reasons may 
be adduced as to the difiiculty of making universally true the 
" universal screw " ; but, even admitting the next to impossibility of 
such an accuracy, why then call the screw universal when in reality 
nearly each maker of microscopes in London has his own gauge of 
the " universal screw " ? It would be also a great convenience to have 
a universal gauge for the sub-stage fittings, eye-pieces, &c., so that 
the apparatus of any one maker should fit the microscopes of the 
others. At present there is a great discrepancy in the diameter and 
length of microscope-tubes and the gauge of sub-stage fittings of 
some makers, compared with those of others. Why not make these 
also universal ? 
Gilded Glass in the Construction of the Camera Lucida.— It is 
known that the construction of the camera lucida is founded upon the 
simultaneous perception of two images — that of the object, and that of 
the pencil. Various means have been employed to arrive at this 
result. In that of Soemmering it is a metallic mirror smaller than 
the pupil ; that of Amici is constructed on the principle of reflexion 
on a plate with parallel faces ; that of Wollaston, at present most in 
use, consists in a prism, of which the edge, dividing the pupil in two 
parts, permits the object to be seen by the upper half, and simul- 
taneously the pencil by the lower portion. In all these systems the 
fusion of the images is somewhat difficult to seize, especially for 
