270 C. S. Hastings— Optical Constants of Glass. 
ever, to much less accordant results. Rudberg, in investigating 
the optical properties of several crystals, found that in arrago- 
nite and quartz, the variations in density and refractive index 
are in the same direction, but that with -calcite the case is 
different, the index for the ordinary ray seeming inde- 
pendent of changes in tem erature, and for the extra- 
r 
an elegant and celebrated method. I shall quote later, some 
- es ees the only quantitative ones which I have been able 
o find. 
The instrument with which the following determinations were 
made, is the large spectrometer by Meyerstein, belonging to 
the Physical Laboratory of the Johns Hopkins University. 
The circle is 12 inches in diameter, divided to 6’, and reads by 
two microscopes to 2”. The probable error of one division is 
1”-48, and its larger periodic errors are expressed in the formula 
=2"°386+7''82 sin (2+62° 36’)+2.1 sin (2 z2+157°) 
+432 sin (3 2+328°)+0’-46 sin (4 z+122°) 
N being the corrected and z the immediate reading. 
Much labor was expended in putting the instrument in so 
satisfactory a state, for in order to secure uniformity of reading 
it was found necessary to re-grind the axis. The collimating 
telescope too, was, by inexcusable carelessness in construction, 
directed nearly one-fourth of an inch away from the axis, thus 
vitiating all determinations of double deviations by the intro- 
duction of unknown errors of aberration in the object glass. 
rea 
‘two verniers to single minutes of are. This platform turns 
independently of the large circle with inconsiderable friction, 
and upon it is placed the prism to be studied. The methods 
adopted to adjust the instrument and prism, and to measure 
the angle of the prism were exceedingly accurate and perhaps 
& proper place for description here. 
