330 Transactions of the Boyal Microscopcal Society. 
there are four screws in the brass angle piece, connecting the prism 
with the slide, by which adjustment in two directions may be ob- 
tained ; one renders the edge of the prism parallel with the slit, 
and the other renders the base of the prism parallel with the 
internal platform of the instrument. 
Quite recently another method has been used in comparing 
spectra together, and I have within the last week made this the last 
addition to my instrument. Instead of having the spectra one 
above the other, in this arrangement they are actually superposed 
so that the whole of the field is occupied by each of the two, or by 
withdrawing the reflector a little, one half the field displays the 
spectrum from the stage of the microscope, and the other half this 
spectrum combined with that from the comparison stage. 
This result is obtained in the most simple manner. To the 
end of the prism is cemented a small bit of glass with parallel 
surfaces in such a way that it forms, as it were, a continuation of 
the hypothenuse of the triangle of the comparison prism. It will 
be seen at once that it acts precisely in the same way as the glass 
in Dr. Beale’s neutral tint reflector ; some of the rays from the 
stage of the microscope pass through, and some of the rays from 
the comparison stage are reflected at right angles, and so join the 
path of the former. 
The comparison stage is, I think, a much better form than the 
one in ordinary use ; it consists of a flat plate with two hard white 
metal springs, beneath which the usual microscopic slide may be 
readily inserted, or in the same manner the tube carrier. This 
consists of a strip of brass, to which two curved springs are attached 
for holding in position the tubes in which liquids for spectroscopic 
examination are generally sealed. 
To save the expense of a second spectroscope, I had an addi- 
tional tube made with an adjustable slit to take the same direct 
vision prisms, the whole closing in a case for the pocket, which is 
all that is required for the preliminary examination of many objects, 
or to ascertain the presence of certain absorption lines in the solar 
spectrum caused by vapour in the atmosphere, which, according to 
Professor Piazzi Smyth,* is a far more delicate means of foretelling 
rain than the variations of the barometer. 
I cannot conclude without expressing my indebtedness in these 
matters to Mr. Bilger for the readiness with which he carried out 
my suggestions, and for the practical hints he has given me ; but, 
above all, for his kindness in placing his valuable collection of 
prisms at my disposal, and his permission to make use of his work- 
shop and tools for any experiments I might wish to undertake. 
* ‘ Astronomical Eegister,’ Sept. 1877 ; ‘ Journal of the Scottish Meteorological 
Society/ vol. v. p. 84. 
