424 
MR. R. T. GLAZEBROOK ON DOUBLE REFRACTION 
to the position of minimum deviation, forming an arithmetic progression of which the 
common difference is 4°. The prism was then reversed so that the face of incidence 
became that of emergence, and another set of results obtained, extending from minimum 
deviation to nearly grazing incidence on that face. 
Each set of experiments was taken twice, and only in two or three cases were the 
differences between the results of the two measurements, usually made on different 
days, greater than 20". In about 18 per cent, of the measurements the differences 
amounted to 20", in the rest it was less, so that in comparatively few cases is the 
difference between the mean and an extreme observation as great as 10". 
The spectrometer was the same as that used in the experiments with arragonite, 
and was kindly lent me by Professor Stokes. The method of taking the measure¬ 
ments and the means adopted to secure the parallelism of the edge of the prism and 
the axis of rotation of the telescope are described at length (Phil. Trans., 1879). The 
collimator and telescope were focused for parallel rays by means of a method suggested 
by Dr. Schuster (Phil. Mag., February, 1879). 
The focusing was done once for each prism, and remained untouched during the 
experiments with that prism. All the adjustments were made for the red hydrogen 
line C. When the rays from this line were parallel no appreciable alteration was 
required to render the sodium rays parallel. 
The other hydrogen rays F and g were very nearly parallel, but probably not 
quite so. 
The experiments were performed in the spectroscope room at the Cavendish 
Laboratory, which was kindly placed at my disposal by Professor Maxwell during 
February, March, and April of the present year. 
The value given for the angle of the prism is in each case the mean of 10 measures, 
no two of which differed by more than 20". 
In the course of the preliminary work I found that variations in temperature of 5° 
or 6° C., to which the room was subject during the months of February and March, 
produced a very appreciable effect in the value of the angles between some of the 
faces. In making the final measurements, therefore, I was careful to keep the room 
at a nearly constant temperature of about 13° C. by means of a gas stove. 
For each position of the prism an observation of the deviation of each of the four 
rays C, D, F, g was taken so that there are four values of deviation, corresponding 
respectively to these four rays, to each angle of incidence. 
Tables I., II., III., and IV. give the results of experiment for the red line C of the 
hydrogen spectrum in the four prisms. 
The error in the result, due to an error in one of the observed quantities, is greatest 
near the position of minimum deviation. If we assume an error of 10" in the values 
of the angle of incidence and the deviation taken so as to produce the maximum error 
in the result, that error amounts to about ‘00005 when a maximum. The probable 
error of the experiments is considerably less than this. 
