444 ME. J. N. LOCKYEE ON SPECTEOSCOPIO OBSEEYATIONS OE THE SUN. 
the upper and lower portions admit light from two other parts of the sun reflected on 
the slit by means of combinations of prisms. 
The slit used in ordinary investigation is similar to that usually employed. 
The train of prisms, the collimator, micrometer, and observing telescope are shown 
in fig. 3 on a larger scale. The prisms are fixed to a plate which it is possible to rotate 
slightly on its axis, and this, after the more obvious precautions have been taken, consti- 
tutes one of the most important adjustments. 
The collimator is furnished not only with the usual focusing-screw, but with addi- 
tional rackwork, which permits of the slit being placed in the image, the colour of which 
is coincident with the arc under examination. It is only by paying infinite attention to 
this point that any good results can be obtained ; and when this is done, and the atmosphere 
is pure and calm, the interval between each of the higher cloud-domes on the sun shows 
an approach to the spot-spectrum, and the spectrum is a mass of horizontal lines. It is 
rare, however, that the atmosphere is steady enough to show this effect in its greatest 
perfection. 
The principal point about the micrometer-arrangement is, that the micrometer-head 
is read by a little telescope, the eye-end of which is immediately above that of the 
observing telescope ; this saves much time, as the position of the observer is not dis- 
turbed when readings are necessary. I have also found it convenient to supplement this 
arrangement by an ordinary eyepiece-micrometer for differential measures of small range. 
I cannot allow myself to conclude this short description without testifying to the great 
perfection of the optical finish of the new spectroscope, which enables me to obtain a 
solar spectrum of the utmost purity. My thanks are due to Mr. Beowning for coming 
to my aid as he did, and employing his well-known skill in the construction of an instru- 
ment at once so unique, portable, and powerful — a task accomplished at last with results 
most satisfactory to myself and creditable to him. 
