MON-OCHROMATIO LIGHT OF ANY DESIRED WAVE-LENGTH. 
933 
Use of the Instrument in the Determination of Re fractive Indices. 
The instrument now described is admirably adapted for supplying the monochro¬ 
matic light necessary for refractive-index determinations, either by the method of 
refraction, or by the method of total reflection. The results in either case are, to 
say the least, quite as accurate as are aflbrded by the direct employment of litliiura, 
sodium, and thallium flames, or the light from incandescent rarefled hydrogen ; and 
the observations are immensely facilitated by the much more brilliant illumination of 
the images of the slit, and the better definition of the limiting line of total reflection, 
and by the ease with which the change from one wave length to another can be 
effected. Moreover, the observations may be supplemented, as in the case of optic 
axial angle determinations, by observations for as many other wave-lengths as it may 
be considered desirable to employ. 
Fig-. 7. 
For the determination of'the minimum deviation of rays refracted by prisms, 
furnished by suitably inclined existing faces upon the crystal, or prepared by grinding, 
the disposition is shown in fig. 7. The highly accurate and in every way admirable 
horizontal circle goniometer, reading to thirty seconds of arc, constructed by FuESS 
