284 Dr. R. A. Houstoun on a New 
being each 2 cms. long. It is cemented to a prism of Iceland 
spar, BDEC, cut with its axis perpendicular to the plane 
of the paper. The angle D is 127° 12', E is 115° 49', and 
BCE is 36° 44'. 
The action of the prism may be better understood by 
considering the beams of light to go in the reverse direction 
— from the object-glass of the collimator to the slit. The 
beam qr is broken into two by the Iceland spar prism, cd 
being the ordinary beam and ah the extraordinary. The beam 
pq is broken into two, but only the extraordinary ef emerges, 
the ordinary being totally reflected at the surface CE. The 
beams ef, cd meet 15 cms. out in an elliptical spot of light 
measuring 2*0 by 2*4 cms., the long axis being vertical. 
The beam ab is quite 2 cms. clear. 
If now we have as source of light an incandescent mantle 
behind a screen, with an aperture at the proper place not 
much larger than 2'0 by 2*4 cms., and if we look into the 
eyepiece, we see two spectra one above the other and polarized 
at right angles to one another. The ordinary component of 
the lower beam misses the slit entirely, the extraordinary 
component of the upper beam misses the object-glass of the 
collimator. 
The prism was fixed in a rectangular brass cell open at 
the ends with the edge A protruding. The brass cell was 
mounted in a short piece of brass tubing with a milled head, 
which turned inside another piece of brass tubing fixed in 
front of the slit. The prism could thus be rotated about the 
axis of the collimator — the only adjustment necessary. The 
inner tube containing the cell could be removed when re- 
quired, the outer tube remaining fixed to the slit. When it 
was returned to its plane the milled head prevented it sliding 
in too far and damaging the edge A. The prism was attached 
to one of W. Wilson's well-known College spectrometers. 
All that is then required to complete the equipment of the 
instrument as a spectrophotometer is one of the nicol-prism 
polarizers with a divided circle, which is supplied by the 
makers to fit over the object end of the collimator. This is 
preferable to the object end of the telescope as it avoids the 
depolarizing effect of the dispersion-prism. The eyepiece 
supplied with the instrument for ordinary use magnifies, how- 
ever, too strongly, when it is used as a spectrophotometer. 
Much the better place for the nicol, however, is the eye- 
piece. In spectrophotometry it is necessary to waste as little 
light as possible. The object-glasses of collimator and 
telescope had a clear aperture of one inch. A nicol with an 
aperture of one inch would be too expensive, and makers 
