138 
Proceedings of the Royal Society of Edinburgh. [Sess. 
off on a milled head. To an eye looking into the telescope two spectra 
divided by a sharp vertical line were visible, and the intensities of these 
spectra were matched for any wave-length by rotating the divided circle 
and polarising prism in front of the slit. The reading of the circle could 
be seen by the observer reflected in a mirror inclined above it without his 
having to rise from his seat. This mirror is not shown. 
The glass troughs containing the solution and the water against which 
it was balanced were placed side by side upon the ledge to the extreme 
right in front of the glass and spar prism, a vertical sheet of zinc with two 
apertures or windows punched in it intervening to cut off extraneous light. 
The source of light was a screened incandescent mantle. 
The instrument therefore resembles the older one except that the 
horizontal and vertical planes are interchanged, and the rotating prism is 
placed in front of the slit instead of in the eyepiece. This necessitates the 
introduction of the lens to get the vertical edge of the prism in focus at 
the same time as the slit. 
There was more difficulty in making the preliminary adjustments than 
in the case of the former instrument ; and owing to the absence of an 
arrangement for screening off the sides of the field of view, determinations 
at the ends of the spectrum were not so accurate. Owing to a grating 
being used instead of a prism, the field was not so bright as in the former 
instrument, and this also made work at the ends of the spectrum difficult. 
On the other hand, the instrument was compact and very rapid to work. 
The method of taking readings and reducing results was the same as with 
the older instrument. It was found convenient to keep to a definite set of 
wave-lengths, corrected for small errors of the scale and for the width of the 
slit ; the range was usually from 432 to 667 /ul/ul . In accordance with the 
design of the prism, the source of light had to be placed about eight inches 
away from it. 
