924 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
wave-lengths through the exit slit, in order that light of any wave-length may at any 
subsequent time be reproduced, is carried out as follows ;— 
The reflecting mirror is attached in fi’ont oTthe slit of that optical tube which is to 
be used as collimator, and the eye-piece holder carrying one of the eye-pieces, perferably 
the second one magnifying four diameters, is attached in front of the slit of the other 
optical tube so as to convert the latter into a telescope. Sunlight is then reflected 
along the axis of the collimator, and the jaws of the slit of the latter are approached 
until the best definition of the solar lines is obtained upon looking through the 
telescope and arranging the prism and telescope for minimum deviation of the 
refracted rays. The exit slit in front of the eye-piece should be opened wide in order to 
obtain an unobstructed view of tlie whole field, when about one-half of the spectrum 
is included in the field at once, and by moving the telescope the whole spectrum 
may be observed. It is manifestly impossible, however, with the prism set for 
minimum deviation to bring the whole of the colours of the spectrum into the centre 
of the field by rotation of the prism, the telescope being fixed. But if while the 
prism is arranged for minimum deviation the telescope is moved round some little 
angular distance, so as to pass the whole of the spectrum from red to blue, and is fixed 
in a position when the centre of the field is just past the extreme violet, a wave 
length in the ultra-violet being thus set centrally at minimum deviation, it will then 
be possible by movement of the prism in either direction to bring the whole of the 
colours of the spectrum in succession past the vertical diameter of the field. That 
one of the tvm directions of movement of the prism is chosen in which the greater 
loss of light by reflection from the receiving surface of the prism occurs when the red 
end of the spectrum is brought to the centre of the field, and the lesser loss when the 
feebler illuminating violet end is central ; by this choice the illuminating values of 
the different colours are rendered less unequal than they usually are wuth a fixed 
prism, while if the other direction is chosen the inequality is intensified. The 
definition of the solar lines for this setting is still admirable, the focussing being- 
achieved by means of the milled head in connection with the rack and pinion. 
Having firmly clamped the telescope to the fixed lower circle, the solar lines for 
which it is desired to record the prism circle readings are well noted while the exit 
slit is still widely open. The jaws of this slit are then f)ronght so closely together 
that the Interval between their knife-edges, which are clea) ly defined by the eye-piece, 
is only very slightly greater than that between the two sodium D lines, that is, not 
greater than two-thirds of an apparent millimetre. The desired solar lines are then 
in turn brought, by rotation of the tangent screw of the fine adjustment, midway 
between the two edges of the slit, which thus act like a pair of vertical cross-wires. 
The exact distance of the jaw^s apart is of no consequence so long as it is sufficiently 
small to permit of accurate adjustment of the lines to the central line between them, 
as the jaws move equally on each side of this central line. If the whole width of the 
receiving slit is employed the lines are very slightly curved as previously stated, but 
