922 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
sacrificing the light by reflection. The heavy colourless glass supplied by Messrs. 
Chance satisfies these conditions, its dispersion being higher than that of ordinary 
flint, while not too great to be a disadvantage. Very great care has been taken to 
make the two utilised surfaces truly plane, and at right angles to the base. The high 
cost of so large a prism of heavy glass, truly worked, is amply compensated by the 
advantage gained in the large amount of light transmitted. The definition of the 
solar and metallic lines afforded by this prism and the lens combinations previously 
described, is of very high quality up to the extreme end of the violet. With the 
lowest power eye-piece, magnifying two diameters, the two D lines of sodium are 
clearly separated ; the second eye-piece, magnifying four diameters, exhibits them 
half a millimetre apart; and the third eye-piece, magnifying six diameters, separates 
them by quite an apparent millimetre. 
For convenience in viewing the solar lines a small mirror is provided, which is 
capable of the four motions necessary for the reflection of sunlight along the axis of 
the collimator. Its carrier is attached to an annulus furnished with a milled flange, 
and carrying a screw thread upon its inner surface of the same pitch as that of the 
eye-piece carrier, so that it may be firmly screwed to the projecting annulus, m in 
fig. 2, of the slit frame of that optical tube which is chosen for convenience as 
collimator, just as the eye-piece carrier is screwed to the similar annulus of the other 
optical tube which it is desired to use as telescope for the purpose of observing the 
solar lines. The mirror and its carrier are represented at the left-hand corner of the 
base-board in fig. 1. 
The ground glass screen which is employed for the purpose of diffusing the line of 
monochromatic light escaping from the exit slit, in order that the whole field of the 
observing instrument may be evenly illuminated, is conveniently held in a small carrier 
forming an attachment in front of the exit slit similar to that just described. This 
attachment is shown at the right-hand corner of the base-board in fig. 1. It consists 
of an annulus provided outside with milled flange and inside with a screw thread 
capable of engaging with that upon the projecting annulus of the slit frame, exactly 
similar to that which carries the adjustable mirror; to the arm carried by the annulus 
is fixed at right angles, tliat is horizontally, a strong rod of square section and 
2| inches long. Upon this rod slides easily a short tube of similar square section and 
bore, which supports, by means of a short upright, the tube of two inches diameter 
and two inches length which carries within it the ground glass screen. The slider 
can be fixed in any position along the rod by means of a clamping screw. Two 
ground glass screens are provided, one of the texture of fine photographic focussing 
glass, and the other still more finely ground. They are mounted in circular metal 
frames like lenses, and the frames are of such a size as to be capable of sliding fairly 
tightly in the carrying tube. In the illustration one screen is represented in position 
inside the tube, and the other lies on the base-board just behind the eye-pieces. 
Either screen may l^e employed according to its ascertained suitability for use with 
