359 
introductory remarks, but reserve an account of each par- 
ticular process until I come to those practical questions 
which require its adoption. 
In the first place, I beg to refer to my papers “ On a 
Definite Method of Qualitative Analysis of Animal and 
Vegetable Colouring Matters,” 1 and “ On the Colouring 
Matters of Blue Decayed Wood,” 2 since much that I shall 
describe will be a continuation and application of what they 
contain. I shall make use of the same scale for measuring 
the position of the absorption bands, and the same notation 
and symbols to express the varying power of absorption in 
different parts. This scale is an interference spectrum with 
dark bands, which divide the whole visible spectrum into 
twelve portions of equal optical value ; and it is so adjusted 
that the sodium line D is situated exactly at 3}. On this 
scale the principal Fraunhofer lines occur as follows : 
The only objection to this scale is that its accuracy is 
dependent on the careful preparation of the plate of quartz. 
I have myself made several perfectly alike and accurate, and 
nothing but proper care is required. When once accurately 
made, no scale could be more convenient, since the bands 
occur at equal optical intervals over the whole spectrum, and 
are quite distinct when the slit is opened rather wide, which 
is sometimes a great advantage. In these two respects it is 
far superior to any other scale hitherto proposed. 
The intensity of absorption will be expressed by means of 
dots, hyphens, and dashes ; thus : 
When these symbols are printed between numbers they sig- 
nify that there is a more or less strong absorption between 
those points of the spectrum as measured by the above- 
named scale ; whereas when they are printed under numbers 
they signify that there is a distinct absorption band, of the 
intensity expressed by the different symbols, the centre of 
which is situated at that point of the scale indicated by the 
figures. This latter method is extremely simple and conve- 
nient, and often serves to express all that is requisite. It 
1 ' Proceedings of the Royal Society,’ 1S67, vol. xv, p. 432. 
3 ‘ Quarterly Journal of Microscopical Science,’ January, 1869. 
VOL. IX. — NEW SER. A A 
A 
E 
B ... 1L C ... 2f D. ... 3i 
b ... 6* E ... n G ... lOf 
Not at all shaded 
Very slightly shaded 
Slightly shaded 
Moderately dark 
Very dark 
Double hyphens. 
Single dash. 
