MONOCHROMATIC LIGHT OF ANY DESIRED WAVE-LENGTH. 
931 
distant from the centre of the apei-ture. The lines are then clearly focussed, and a 
tracing of them made by means of a fine blacklead pencil upon the ground surface 
(which is nearest the observer) of the glass itself. Slits corresponding to the traced 
lines can then be cut as finely as desired in a diaphragm of stout tin-foil, the relative 
openings of the slits being varied slightly in the inverse proportion to that of the 
relative illuminating power of the light which is to pass through them. The tin-foil 
diaphragm is conveniently supported in a slider of hard wood, whose edges are 
bevelled, and which is likewise of the right size and sufficiently thin to slide into the 
dove-tailed recess after removal of the slip of ground glass. By employing a very 
hard variety of wood, such as box-wood, hard olive, or ebony, the slide may be con¬ 
structed out of c^single piece, and is then quite as durable and rigid as metal, and is 
preferable to the latter, as it is incapable of wearing the bevelled guides of the recess. 
It is shown in fig. 1, leaning against the spirit-level. It is furnished with two guiding- 
grooves, cut with a fine fret-saw, along which the tin-foil diaphragms are capable of 
sliding, and its central portion is cut away in order to permit the spectrum to impinge 
directly upon the diaphragm. By making several such diaphragms for the combina¬ 
tions which are likely to be required, and indelibly numbering them so as to ensure 
identification, any desmed one may be placed in the frame at any time to furnish light 
of the required composition. The accuracy with which the slits have been cut should 
be tested at the time they are made, by placing the lowest power eye-piece in position 
in front of the slit-frame and observing whether the focussed lines, from a tracing of 
which the slits were cut, can be brought by suitably moving the slider to simul¬ 
taneously occupy the centres of the slits. In order to avoid having subsequently to 
set the selected diaphragm to the correct place in the spectrum by the aid of Fraun¬ 
hofer or metallic lines, the following simple device is adopted :— 
A pair of fine marks, forming a continuation of the same vertical line as that in 
which the knife edges of the slit-jaws meet, have been made on the rigid framework 
of the slit-box immediately above and below the slit. A similar pair of marks are also 
made upon the guides of the wooden slider. After the diaphragm has been tested as 
above with the eye-piece and found to be satisfactory, the slider is adjusted in 
the recess so that the marks upon it are in the same vertical line with those upon 
the rigid framework. Maintaining the slider in this position the diaphragm, if not 
already so arranged, is then moved in the guiding-grooves of the slider until the same 
position as before is attained, when the lines are seen simultaneously focussed in the 
centres of the slits. A vertical mark is then likewise made upon the tin-foil dia¬ 
phragm by means of a fine needle, exactly in the same line as the other marks. The 
leading of the prism circle is observed while so adjusted, and recorded. It is then 
only necessary at any subsequent time, when it is desired to employ that particular 
diaphragm, to place it in the slider with the mark i?i line with the two marks on the 
latter, then to place the slider in the recess so that these marks are also in line with 
those on the slit-frame, and to set the prism circle to the reading previously recorded 
6 c 2 
