New Form of Pliotomicrographic Camera. By E. B. Stringer. 179 
only adjustment that has to be made with each fresh exposure is the 
centering of the light itself (which is for this purpose provided with 
very sensitive centering screws), and the adjustment of its distance 
from the condensers, which will he found to vary a little when 
different objectives and suhstage condensers are used. The small 
sheet-iron lantern slides in grooves on each side of the optical bench, 
and, when slid backwards, gives easy access to these screws. 
Finally, it must he said, that to employ this illuminating arrange- 
ment to the greatest advantage, it is necessary that the suhstage 
condenser be proportioned in power as well as in aperture to the 
objective. The more nearly the power of the substage condenser 
approaches that of the objective, the wider can the large iris he 
opened, and the greater is the intensity of the illumination ; but it is 
the intensity alone which is affected ; the aperture of the substage 
condenser, whatever it may happen to be, remaining unaltered. The 
best results appear to be obtained when the condenser is about one- 
half the power of the objective. 
If this proportion he pretty closely maintained, and the other 
conditions are fairly equal, as is often the case in the general run 
of work, the exposure varies but little with varying magnifications, 
averaging about two seconds with a light yellow screen and a plate 
of medium rapidity. 
The apparatus was made for me by Messrs. Watson in the summer 
of 1896, and I must, in concluding, express my thanks to them for the 
care and thoroughness with which they have carried out my design. 
