MONOCHROMATIC LIGHT OF ANY DESIRED WAVE-LENGTH. 
919 
means of a simple locking arrangement, shown in fig. 2. In the slide c an L-shaped 
groove g is cut, in which slides a j^in h, carried by the jaw. This pin is not fixed 
into the jaw itself, but into a short slider Jc, furnished with bevelled edges, which is 
capable of sufficient vertical motion between two guides I (one edge of each of which 
is likewise bevelled, so as to form together a dove-tail in which the slider is supported), 
to enable the pin to be raised to the level of the horizontal part of the groove, when 
the jaw may be readily withdrawn. In order to place the jaw in position again it is 
only necessary to slide it into its dove-tailed recess, until the pin reaches the end of 
the horizontal part of the groove; it is made to slide quite smoothly, without 
jamming, by fitting a short curved spring in a suitable niche in the upper horizontal 
edge, so as to press slightly against the upper guide ; the slider k is then lowered so 
as to bring the pin down the vertical portion of the groove, when the jaw will be 
firmly locked to the slide c. The pin and groove are so well fitted that precisely the 
same position is always occupied by the jaw, when locked, with respect to the slide. 
When it is desired to utilise the above arrangement for the purpose of replacing the 
single slit by two or more, it is found more convenient to construct them permanently 
in a simple but highly accurate manner, which will be described under the heading, 
“ Mode of Production and Use of Composite Light,” than to employ an elaborate 
metal arrangement of several movable and adjustable slits, such as is so admirably 
adapted to Abney’s form of apparatus, but upon which inconvenient limitations are 
necessarily imposed, and which would require re-adjusting by means of the solar or a 
metallic spectrum for every variation. 
The full wfidth (using this term in its current sense denoting the longest dimension 
of the opening) of the slit of each optical tube is one inch ; this relatively large width 
is not intended to be generally utilised, but is provided for use in observations with 
imperfectly transparent crystals, when, subject to limitations to be presently specified, 
it is of great advantage as it transmits a correspondingly large amount of light. It 
is of course impossible with a slit of one inch width to avoid a slight curvature of 
the spectral lines, W. H. M. Christie”' has shown that this curvature cannot be 
eliminated by adjustment of the prism or prisms, and that it increases with the 
number of prisms ; hence it is least with a single prism as used in this arrangement. 
The lines are slightly concave towards the normal to the surface of incidence of the 
prism. Particular care has been taken in the setting of the lens combinations that 
such curvature should not be accentuated by any slight want of parallelism in the 
incident light. The slight deviation from perfect monochromatism in the light 
issuing from the exit slit, consequent upon this slight curvature of lines of light 
vibrating with the same wave-length, is found in practice to be no detriment what¬ 
ever in the measurement of the optic axial angles of crystals whose dispersion of the 
axes for the red and blue does not exceed 5°, and the brilliant illumination of the 
field by use of the one-inch slit is a very great advantage when dealing wfith sections 
* ‘ Roy. Astron. Soc. Monthly Notices,’ 1874, 263. 
