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SUMMARY OF CURRENT RESEARCHES RELATING TO 
supported by four legs, with a focusing screen at the top, and at the 
lower end, a leather bellows, by means of which connection is made 
between the camera and the Microscope. On one side of the camera 
body, a large hinged door having rabbeted fittings is placed, and 
on opening this the head of the worker can enter the camera, and the 
eye be placed to the tube of the instrument to make the adjustments ; the 
door is then closed and final focusing is done on the screen at the top 
of the camera. Messrs. Watson state that the advantages are, that all 
the motions of the Microscope are under the direct control of the hand, 
and that it can be at once placed over the Microscope, and a photograph 
taken, without the re-adjustments necessitated by the horizontal form of 
camera. 
(5) Microscopical Optics and Manipulation. 
Microscope Tube-length and Resolving Power.* — Mr. H. G. 
Jameson writes : — “ In the discussion of the relative merits of the English 
and Continental tuhe-lengths, one argument in favour of the English 
tube seems to have escaped notice, namely, that it gives, with any 
individual lens, a distinct advantage in resolving power. Taking the 
values for the different tube-lengths recently adopted by Prof. Abbe, the 
gain in resolving power for the English tube with a (true) 4 in. object- 
glass, is as much as 43 per cent., with a 2 in., 13 per cent., and with a 
1 in. 5 per cent. With a 1/4 in. the difference falls to 1 per cent , and 
with higher powers it becomes imperceptible, but still a gain in definition 
of 5 per cent, with a 1 in. o.g. is not to be despised. 
The advantage of the long tube may easily be observed practically 
by taking an ordinary optician’s 4 in. objective (probably really about a 
3 in.), and inserting a diaphragm behind the lens about 1/4 in. diameter, 
and examining the 1/100 mm. lines on a stage micrometer — it will be 
found that these are distinctly separable with a 10 in. tube, but vanish 
when it is shortened to the Continental standard. 
This seems at first sight contradictory to Prof. Ahbe’s formula, 
which makes the number of lines per inch resolvable by any lens 
But we have only to examine carefully the definition of 
numerical aperture, viz. N.A. = n sin u, to see that N.A. is itself a 
variable quantity. The matter may be simplified by leaving ont the 
term n, that is, by considering only non-immersion lenses, in which case 
N.A. = sin u. Let R, = the semi-diameter of the lens,/ = its focal 
length, and T = the optical tube-length. Then, so long as the lens is 
focused at its true focal length, and the rays issue parallel beyond it, 
R 
sin u = — . But in practice, of course, the lens is withdrawn farther from 
the object, so that the rays converge above to form an image at a distance 
depending on the tube-length. Call this anterior conjugate focus /. 
T / 
Then /' = — A_. And, therefore, in the practical use of the micro- 
scope, we get the modified expression, depending upon the value of 
* Engl. Mecli., liv. (1892) p. 489. 
