8 O. N. Rood on Photometric Experiments. 
Mode of determining the indices of refraction. 
In my experiments on transparent substances, oo where 
it was possible prisms of the substance were groun the in- 
dex of refraction of the sodium line determined as sia} with a 
graduated circle, collimating and observing telescope. In the 
articular experiments detailed at the termination of this article, 
the plates of glass used were so thin that it was not practicable 
to grind from them prisms, and for all such cases I contrived, 
tested and used two somewhat new modes of procedure, as 
neither the method of the Duke de Chaulnes (alteration of the 
focus of a microscope,) nor that of Bernard (displacement of an 
image viewed panely through a plate,) were here found t 
give > reliable 
1. A minute angular fragment of the glass to be experi- 
mented on was placed in a cell, on a glass slide, like those used 
for mounting microscopic objects, and surrounded with a mix- 
ture of “body-sperm-oil” and oil of cassia, the proportions be- 
ing varied the refraction of the glass for the sodium line had 
been exact. vo Se aaa by the oil. Olive oil became turbid 
when mixe e oil of cassia and hence could not be used. 
The mode of a caivg the refractive power of the mixture of 
the oils and glass was as follows: at the distance of half an inch 
below the level of a microscope stage was a fine slit, cut in tin- 
foil which had been pasted on glass; the microscope was fo- 
cussed on this, a sodium flame being used to illuminate it; the 
cell with the oil and fragment of glass was then placed on the 
stage of the microscope, and moved so that the light from the 
slit passed through the angular fragment, when it would happen 
that the line of light would be refracted to the right or left hand 
according as oil or glass predominated in re ve power, which 
made it Sectonily evident whether sperm oil or oil of cassia was 
needed. number of experiments were made to test this 
method, eich was found to answer well, the index of refraction 
as determined by prism corresponding ‘with that obtained by 
the use of the ent: so in the case of a sample of crown 
gies s a PE We prism gave the index of refraction as 1526, 
while by the new method it was found to be 1529. This 
method, however, is capable of still more accurate results, as in 
the above mentioned experiments, the compensation was pushed 
only far enough to answer my immediate purpose ; that is to 
say, the a may be considered to consist of one large tri- 
eee P with a moderate angle, and a number of smaller 
tion no oa taking place, and the werfully on the 
light coming from the slit, still furnishi power : es having 
