282 Transactions of the Boyal Microscopical Society. 
over the tube which contains the objective, which has a hole, the 
one-sixteenth of an inch in diameter, cut in it. By this arrange- 
ment all extraneous light is prevented from passing up the body 
of the microscope, except what passes through the object. I 
have tried examining objects both with and without this small piece 
of apparatus, and am quite convinced that unless this precaution be 
attended to, a false result is frequently obtained ; this, however, I 
am quite sure of, that it tends to assist the observer greatly in 
making correct measurements. 
No. 1. See Plate CLXI., Fig. 2. 
Oxalate of Chromium and Soda. 
This solution is of a blackish purple colour, and is remarkable 
for the sharp narrow band below B, in the red ; the others in the 
yellow and violet I have discarded as not being serviceable to my 
requirements. It is almost opaque by daylight, and shows no 
bands, it must therefore be viewed by the aid of a lamp. 
M. 
A. 
Obseryations. 
23-407 
Middle 
711-75 
(Yerj black band, | of a division in size, 
\ ends, sides, and centre equal. 
No. 2. See Fig. 3. 
Sodium 
is an incandescent spectrum, and expresses D solar or the line 
produced by the combustion of sodium in the Bunsen or spirit- 
lamp flame ; it is easily detected owing to its position, which is in 
the centre of the yellow. 
M. 
A. 
Observations. 
22-875 
Middle 
589-2 
( Bright well-defined band, in the yellow, 
\ size about J a division. 
No. 3. See Fig. 4. 
Nitrate Didymium (or Bidymium Nitrate). 
Owing to the extreme sharpness of the absorption bands in this 
spectrum, it is the one I invariably use as a standard to determine 
the position of others by. 
