jZnSS?rim'] ^oyal Microscopical Society. 125 
without the microscope ; the dispersion obtained with this small in- 
strument is remarkable. A description of it appears in the August 
number of the Journal, page 65. Mr. Crookes exhibited at a late 
meeting of the Koyal Society a spectroscope adapted to use with the 
binocular microscope, which he said had been devised to obviate the 
disadvantages of the ordinary micro-spectroscope. 
The instrument is described in the June number of the 
Journal, page 371. I would observe, it appears to me to differ 
in no very essential particular from one constructed by Mr. Brown- 
ing two or three years ago, and which was soon after abandoned. 
I am not at all surprised that it should have been thrown aside, as it 
occurred to me at the time that its disadvantages counterbalanced its 
advantages. The defects of that of Mr. Crookes are that it requires 
the addition of a substage to carry the condenser, slit, and reflect- 
ing prism, and also a box to hold the dispersion prisms. The prisms 
and slit being attached to different parts of the microscope, the 
apparatus is more likely to get out of order than that of the Sorby- 
Browning, in which the prisms are fixed in their required and re- 
lative position in the eye-piece. The microscope must be sent to 
the maker of the spectroscope ; certainly a disadvantage, when by 
cutting out a piece of cardboard into which an eye-piece fits is all 
that is required for the purpose of adapting the " direct vision 
Sor by-Browning instrument." 
Mr. Crookes refers to the convenience his form of spectroscope 
possesses over others, in enabling observers to use it with a bino- 
cular microscope ; it should, however, be borne in mind that the 
light is much degraded by passing through a narrow slit, and is 
then spread out by the fan-like action of the prisms, and that 
many lines which may exist in the deep-blue of the spectrum 
will probably be lost or passed over unrecognized, particularly if 
the light be divided between two eye-pieces. Mr. Crookes forgets, 
also, that the best results are those obtained with a limited amount 
oi dispersion. Substances which show several fine absorption bands 
with a low dispersive power, appear to be without lines when a 
higher dispersive power is employed. A solution of chloride of 
cobalt in chloride of calcium will show this most conclusively. 
Not the least important part is, that the whole apparatus will be 
necessarily much more costly. 
An interference spectrum is employed by Mr. Sorby for the 
purpose of dividing the spectrum into regular intervals, for the con- 
venience of facilitating a careful record of the results obtained. 
This, in the form proposed by this gentleman, is not unattended 
with disadvantages, when reduced to practice. If considered neces- 
sary to employ an artificially divided scale for reference, in place 
of the now universally adopted Fraunhofer's lines, it will surely be 
better to make use of a natural material, as Zircon, discovered by 
