422 MR WILLIAM SWAN ON THE PRISMATIC SPECTRA OF THE 
of the readings then gave double the angle of the prism. For, if DGI, FHK re- 
present the course of the reflected rays, since the telescope has been adjusted to 
sidereal focus, GI and HK, must be parallel; and the angle DEF, will obviously 
be double the angle BAC. Now, since DE and EF, are at the times of ob- 
servation successively in the same direction, namely, that of the parallel rays emerg- 
ing from the collimator, it follows that the telescope must have been turned 
through the angle DEF. Hence the difference of the readings is DEF, or twice BAC. 
In order to test the adjustment of the collimator to sidereal focus, [ made two 
series of observations of the angle of the prism given in Table I.; Series I. having 
been made by means of the collimator, and Series II. on a definite point of the 
tower of St Stephen’s Church, distant about 2240 feet, where the parallax due to 
any difference in the directions of the rays incident on the two faces of the prism 
could not have caused an error exceeding 4” in the measured angle. 
These results agree so closely as to show that any want of parallelism in the 
rays emerging from the collimator, arising from want of perfect adjustment to 
sidereal focus, could not have appreciably affected the observations of the absolute 
deviations of the refracted rays. I may also observe, that since, during the ob- 
servations of the carbohydrogen and solar spectra, the whole apparatus remained 
unaltered, any want of parallelism in the rays incident on the prism, whether aris- 
ing accidentally from imperfect adjustment of the collimator, or necessarily from 
the unavoidable want of perfect achromatism in its lens,—for either cause might 
modify the apparent direction of the observed object, if the pencil of rays incident 
on the prism were not accurately cental,—would affect the observed deviations in 
the two spectra alike. The accuracy of the observations, viewed merely as afford- 
ing a comparative view of the relative positions in the scale of refrangibility occu- 
pied by the lines in the two spectra, would thus remain entirely unimpaired. 
I have ascertained, however, by actual experiment, that the observations of 
absolute deviation cannot have been sensibly affected by any want of achromatism 
in the lens of the collimator. Having caused the telescope wires to coincide ac- 
curately with the image of the collimator slit, I illuminated the slit alternately 
with the extreme red and the extreme violet rays of the solar spectrum formed 
by a flint-glass prism. I then found that the image of the slit did not in the 
slightest perceptible degree alter its apparent position; so that, while the illumi- 
nation was changed from red to violet light, the wires continued to bisect the slit 
with perfect accuracy. 
As the spectrum of the Bunsen lamp is so faint that the telescope wires, when 
projected on all but its brightest lines, are invisible, it became necessary to illu- 
minate the wires; but I speedily found that, from the feeble luminosity of the 
spectrum, observations with an illuminated field were nearly impracticable, 
and I was therefore obliged to observe with illuminated wires on a dark field. 
The arrangement for illuminating the wires which I devised is so simple, and 
