432 Transactions of the Society. 
window, so that the table can be easily placed exactly in the required 
position. 
The table itself (fig. 48) is heavy and solid, and stands upon 
three legs, so as to secure an equal bearing. It is at such a height 
that the horizontal Microscope-tube is at a convenient level for 
eye-observation, when the observer is seated, so that all the pre- 
liminary adjustments, as regards cover-correction, &c., can be 
comfortably made, and the illumination regulated, before the 
camera is attached. The base-board of the camera pivots on a 
steady tripod, and can, during this process of adjustment, be swung 
aside out of the way, but be brought round when required, and the 
anterior end of the base-board then fits to the edge of the Microscope 
table. The attachment of the camera to the Microscope is effected in 
the usual manner. For my own work, I find it most convenient to 
use a camera ot fixed length, viz. one metre from eye-piece to sensi- 
tive plate ; but a bellows body, capable of variable extension, can, of 
course, be substituted if desired. The focusing rod disconnects at 
the anterior end of the camera, sliding back off a square pin from the 
portion attached to the Microscope table. It works by means of a 
string, that passes round the milled head of the fine-adjustment ( Fig. 
49). The bar which carries the socket of the substage condenser has 
attached to it a small platform, upon which can be placed a screen 
of dark-blue glass, to subdue the glare for eye-observation, or a small 
cell containing ammonio-sulphate of copper or other solution, for 
producing monochromatic light. 
So far, however, I cannot say that I have experienced any practical 
advantage from monochromatic light. It appears to me that when 
ordinary sunlight is used, the blue-violet rays are so prepotent in their 
actinic power that they do all, or nearly all, the work, and the other 
rays have not time to produce any material effect. The supposed 
advantages of monochromatic light are then practically attained with- 
out any special means, unless, indeed, some special method can be 
devised for working with rays of shorter wave-length than the blue- 
violet ; and any suggestion for accomplishing this I shall be glad to 
receive, and to give it a trial. 
The resolving power of our objectives depends not only upon their 
numerical aperture, but also upon the wave-length of the light used ; 
and the high ultra-violet rays should therefore give a higher resolving 
power than the blue-violet ; but I have not yet succeeded in making 
them operative in practice. 
As regards general manipulation, the only special recommendations 
that I have to make are : — (1) That the cone of illumination should 
always be strictly axial. (2) That the image of the sun should be 
focused exactly in the plane of the object, so that it shows sharp and 
clear on the ground glass when the object is in focus. Clouds close 
to, or passing across the face of the sun, should be seen almost as if 
a landscape lens was being used. (3) That no unachromatized lens 
