1875.] Physics. 405 
these prismatic or inclined mounts, it must be borne in mind that nearly 
every object lies under a different thickness of glass, according to the distance 
from the keen edge; therefore having selected a suitable one, the objective is 
to be carefully adjusted to give proper definition, and a thickness of glass 
over the scale may probably be found that will best suit its correction. For 
dry mountings, the object-glass must be used dry, as water would run in and 
spoil the object. If this is in balsam, of course the immersion system can be « 
employed. The prismatic mounting slips may easily be made by any 
ordinary glass grinder. Thin well-polished sheet glass (such as is used for 
microscope slides) is cut into pieces, three quarters of an inch long by four- 
tenths wide. Eight or ten of these are cemented together with hard Canada 
balsam, and their step-like projecting edges adjusted against a bevel set at the 
desired angle, say 35°. They must be pressed into very close contadt, as it is 
important to have the edges worked fine and clean, and any thickness of 
balsam stratum between them will prevent this, for during the work close 
edges mutually prote& each other. Having prepared a sufficient number of 
blocks in this way, they are cemented on to arunner or metal plate with the 
usual cement of pitch and wood ashes. They are then ground and smoothed 
on a flat metal lap, till all the steps are gone and keen edges shown, and are 
then finally polished. 
A lamp for the convenience of microscopists who are in the habit of using 
their instrument away from home has recently been contrived by Mr. Swift. 
It possesses the great merit of extreme portability in combination with the 
rare merit of perfect efficiency. The dimensions of the cylindrical case are 
As in Use. Packed Ready for Case. 
8} in. by 1} in. The arrangements for packing the lamp are very simple, so 
that no time is lost in setting it up and putting it away. The burner is flat, 
of small dimensions, but the flame is quite large enough, and the light of good 
quality, The chimney is of metal, the lower part being cut. away so as to 
