346 
Construction of Object-glasses. 
[Monthly Microscopical 
Journal, June 1, 1869. 
together to the required angle and polished ; the marginal error 
will be taken up by the lower end of the under-plate. It would be 
impossible to make an acute wedge of this figure in any other way, 
and when separated it will be found to have a knife-edge perfect at 
the extreme. 
Another example may be described, from my practice in making 
the first prisms for the binocular microscope. A, Fig. 2, is an 
end view of the intended prism ; this is supposed to have been a 
block of glass of larger size, with one polished surface cemented 
with Canada balsam on to the guard-plate, B ; the front and back 
reflecting surfaces are then smoothed and pohshed ; these are then 
covered with guard-plates, and the top emergent surface of the 
prism ground off and polished to the dotted line, C, C. It will thus 
be seen that every corner of the prism is protected during the 
working, and is kept absolutely perfect to the edge. The prisms 
were made sufficiently long to be cross cut into three or four. The 
smoothing was performed in accordance with the foregoing direc- 
tions, but the polishing lap was required to be much smaller. The 
one that I employed was only inch in diameter. If a large 
lap is used, the polish is apt to commence on the margins of the 
glass ; and if this is the case a true reflecting figure will never be 
obtained. The polish should begin in the centre and spread to the 
outside. The proper angles for these prisms were set off by a 
graduated steel sector, and as the measurements have to be taken 
from the back of the guard-plates it is necessary that these should 
be exactly parallel ; if not so, they must be ground on the surface- 
laps till all the edges gauge alike. 
I may here remark that I am merely recording what has been 
my own self-acquired practice, and which is perhaps neither the 
most expeditious or easy. My best apology must be that I have 
always secured perfectly accurate results by these methods, and when 
a few only are required I must confess that I do not see a better 
way. But the great demand that has arisen for binocular prisms 
has induced the makers to discover a plan of working them in 
blocks, a number at a time, the particulars of which I do not pretend 
to explain. 
Some very excellent prism work is produced on the Continent, 
and as the mode of polishing is peculiar it may be worth while to 
record it. Chevalier and Co., of Paris, through Messrs. Beck, 
politely sent me an explanation of the process, together with a 
sample of all the grinding and polishing materials used in their 
business. After the surface of the prism is smoothed, a piece of 
very thin, smooth paper (much resembling photographic negative 
paper) is cemented by its extreme ends with a little gum or dextrine 
to the metal lap ; a lump of yellow tripoli (labelled " TripoH de 
Yenise ") is then rubbed dry over the paper, and the prism, also dry, 
