536 
ME. W. HUGGINS ON THE SPECTEA 
by a clock-motion. As even on nights of unusual steadiness the lines in the spectra of 
the stars are necessarily, for several reasons, more difficult of minute discrimination of 
position than are those of the solar spectrum, it is important that the apparatus employed 
should give an ample amount of dispersion relatively to the degree of minuteness of ob- 
servation which it is proposed to attempt. 
In 1866 I constructed a spectroscope for the special objects of research described in 
this paper, which was furnished with three prisms of 60° of very dense flint glass. The 
solar lines were seen with great distinctness. I found, however, that, in order to obtain 
a separation of the lines sufficient for my purpose, an eyepiece magnifying ten or twelve 
diameters was necessary. Under these circumstances the stellar lines were not seen in 
the continued steady manner which is necessary for the trustworthy determination of 
the minute differences of position which were to be observed. After devoting to these 
observations the most favourable nights which occurred during a period of some months, 
I found that if success was to be obtained, it would probably be with an apparatus in 
which a larger number of prisms and a smaller magnifying power were employed. 
The inconvenience arising from the pencils, after passing through the prisms, crossing 
those from the collimator when more than three or four prisms are employed, and also, 
in part, the circumstance that I had in my possession two very fine direct-vision prisms 
on Amici’s principle, which had been made for me by Hofmann of Paris, induced me to 
attempt to combine in one instrument several simple prisms with one or two compound 
prisms which give direct vision. An instrument constructed in this way, as will be seen 
from the following description, possesses several not unimportant advantages *. 
a is an adjustible slit ; b an achromatic collimating lens of 4 - 5 inches focal length ; 
c represents the small telescope with which the spectrum is viewed. The train of prisms 
* [An apparatus in many respects superior to the one here described has been constructed since. — October 
1868 ?] 
