MONOCHROMATIC LIGHT OF ANY DESIRED WAVE-LENGTH. 
937 
ness of the images is always admirable. The ground and polished faces furnished by 
the instrument described in the preceding communication, however, are so plane and 
brilliant that excellent images are obtained without the use of glass-covering plates. 
Having recorded the two pairs of readings of the goniometer-circle verniers for 
the two images afforded by light of the first wave-length, similar observations are 
made for the remaining wave-lengths by setting the prism circle of the monochromatic 
light apparatus to the proper readings. The whole series of observations are then 
repeated with the telescope arranged upon the other side of the direct reading of the 
slit, and the light incident upon the other face of the crystal prism; the mean values 
derived from observations with the same wave-leno-th in the two series are then taken 
o 
as representing the true angular values of the minimum deviation for light of those 
wave-lengths. 
When it is desired to make determinations of refractive index at different 
temperatures the larger goniometer of Fuess, the micrometer of which reads to ten 
seconds of arc, is employed, together wdth the heating apparatus provided with it. 
It is arranged in connection with the monochromatic light apparatus, precisely like 
the goniometer above referred to. The advantage of employing the form of mono¬ 
chromatic light apparatus now described is here particularly evident, for it is possible 
to complete the whole series of observations for any one temperature in a very brief 
interval of time compared with that taken up by observations with flames and 
Geissler tubes, during which the temperature can more easily be maintained constant. 
This larger instrument may, of course, be employed for determinations at the 
ordinary temperature if such a course is considered desirable, but usually the some 
what smaller and much more handy instrument is preferable. 
In order to employ the monochromatic light apparatus for the purpose of the 
determination of refractive indices by means of the total-reflectometer of Liebisch, 
the crystal holder of the goniometer is replaced by the total-reflectometer, which 
may be either the smaller instrument used with the goniometer No. 2a, or the larger 
type employed with the large goniometer No. 1. The crystal plate is gently 
pressed by means of the delicate arrangement provided for the purpose, terminating 
in a series of Cardani rings whose inner disc carries the crystal, against one face of a 
highly refracting heavy glass prism, whose angle and whose refractive indices for 
different wave-lengths are known. Any minute inequalities in the surface of the 
crystal plate preventing absolute contact between it and the ])rism are rendered of no 
consequence by introducing a film of a highly refractive liquid such as a-mono- 
bromnaphthalene between the two surfaces. In the prisms supplied by Fuess witli 
the total-reflectometer the face.upon wdiich the light is to be incident is ground, so 
that the incident light is sufficiently diffused without the necessity of interposing a 
diffusing screen between the source of light and the prism. 
It is only necessary, therefore, to arrange the goniometer so that the ground face 
of the prism receives the monochromatic light issuing from the exit slit. The most 
MDCCCXCIY.-A, 6 D 
