of the Solar Spectrum on the Daguerreotype Plate. 125 
composition, and varying in thickness according to the law 
of the curve represented in fig. 2, where the abscissa being 
the distance from the end of the spectrum to any point in it, 
the ordinate will represent the thickness of the film at that 
point, where it will be observed that in using the word film, 
I by no means mean to imply the notion of its continuity, as 
opposed to that of a scattering over the surface of lamellar 
particles of the requisite thickness. 
10. In what manner we are to conceive the above-men- 
tioned intrusion of a white member at the commencement of 
the series and the further modification of its subsequent 
members, by dilution to a certain degree of the black, and 
deadening the extreme brilliancy of the white of that series, 
seems a point of difficulty which I am not disposed to dis- 
semble. At one time I imagined that it might be accounted 
for by the intermixture of the transmitted or complementary 
series of colours reflected from the silver below, on the hy- 
pothesis of a high absorptive action in the film itself, which 
would progressively diminish the influence of such intermix- 
ture as the thickness increased, so as to allow the higher tints 
their full intensity. I am not aware that the subject of the 
Newtonian rings, as formed by absorptive films, has ever been 
theoretically treated. But it is easy, without going regularly 
into it, to see that, since the intensity of any given homoge- 
neous ray is diminished by absorption in geometrical pro- 
gression while the sum of the thicknesses traversed by it 
increases in arithmetical—and moreover, since the transmitted _ 
series of tints so brought to the eye by reflexion must have 
traversed (by the theory of their formation) four thicknesses * 
of the film, while the interfering ray which produces the re- 
flected series has to traverse only two—therefore the ratio of 
intensity of the transmitted to the reflected tint, in the state 
in which it reaches the eye, will continually diminish in geo- 
metrical progression as the tint ascends in the scale, so that 
the higher orders of tint will be progressively purer, at least 
so far as this cause is concerned. 
But against this there is one obvious objection, viz. that 
the succession of tints as they stand cannot be allowed to 
commence with the zero of thickness and go on according 
to the Newtonian law, unless we assume that the whole extent 
of the spectrum, and not merely its terminal portions, as Dr. 
* [here for simplicity lay out of consideration the circumstance that 
the transmitted tint so conveyed to the eye consists of an infinite series of 
rays all in one phase of undulation, forming a decreasing geometrical pro- 
gression of intensity, owing to haying traversed respectively 4, 6, 8, 10, &c. 
thicknesses, 
