By Pror, EMERSON REYNOLDS, M.D. 9 
I have tried with success a mode of determining small dif- 
ferences with this apparatus which could doubtless be applied 
with advantage in mapping spectra with instruments of high 
dispersive power. 
The glass plate B was removed from its stand and the index 
from the rod 7; I then attached to the latter a cork carrying a 
small mirror placed at a suitable angle. A spot of light was 
reflected from this mirror and made to fall on a screen placed 
several metres away. ‘The relative distances between the mem- 
bers of groups of closely ruled lines (those of the Nitrogen 
Spectrum) were then easily determined in this manner, as the 
actual motion of the needle from point to point was greatly 
magnified. 
The relative positions and widths of the lines seen with the 
instrument are easily laid down on a millimetric scale. I 
have had a number of 200 m.m. scales printed on narrow slips 
of paper ; and the graduations are lithographed on a band of six 
equidistant lines which thus serve for marking off intensities, 
according to Bunsen’s graphic method. One millimetre corres- 
ponds to one unit of the scale on the opal glass screen, and 
consequently to three minutes as read off with the vernier on the 
eraduted arc of the instrument. Differences corresponding to 
1’ can therefore be easily estimated and represented on the milli- 
metric scale. 
But one other practical point need be mentioned. I find it 
exceedingly convenient to mark off on the opal glass scale the 
positions of the more important lines of the elements whose 
spectra are easily obtained with the aid of the Bunsen flame. 
The symbol of the element to which a particular line or band 
belongs is legibly written under the particular point of the scale, 
and connected by a line with the point in question. Identification 
of the bright lines observed in the spectrum of an unknown com- 
pound is thus greatly facilitated. 
I may be permitted to add that the measuring apparatus 
described has been fitted to the spectroscope used in the College 
Laboratory by Messrs. Yeates and Son of this city, whom I have 
to thank for the care and skill with which they carried out the 
details of construction. 
