242 
MR. AIRY ON THE THEORETICAL EXPLANATION OF 
struction of bands is here, as to sense, complete, although the shift of s is the same 
in amount as that which (when taken the opposite way) produces strong bands. 
Fig. 5. contains a representation of the effect of shifting the bands in the direction 
of the shift produced by placing the blue end of the spectrum next to the mica, so 
that the shift 1*2 in the value of s corresponds to 360° of R. Fig. 6. has the same 
with the shift 1*8 in the value of s corresponding to 360° of R; and fig. 7- has the 
same with the shift 2 - 4 in the value of s corresponding to 360° of R. Each of these 
figures has at the bottom a curve showing the effect of the aggregation, supposing twelve 
curves taken (the sum of the ordinates being divided by twelve), and another showing 
the effect supposing twenty-four curves taken (the sum of the ordinates being divided 
by twenty-four), the value of R for the twenty-fourth being double of that for the 
twelfth. The intensity of the bands in the result is here seen well. 
It must be remarked that no sensible error is produced here by making no variation 
in the value of X. For the different kinds of light which are mingled are not those 
from all parts of the spectrum, but those from parts of the spectrum of a very limited 
extent ; and the properties of the difference of quality of the different rays enter, not 
immediately in consequence of the variation of X, but in consequence of the variation 
of R. 
It is also to be remarked that strong bands are produced as well in fig. 5. as in 
fig. 7, although in the former there is a shift of 1*2 in the value of s corresponding to 
360° of R, and in the latter a shift of 2*4 in the value of s corresponding to 360° of R. 
It appears therefore that, with the same spectrum, considerable latitude in the thick- 
ness of the mica is allowable, or considerable latitude in the degree to which the 
spectrum is viewed out of focus. For the same reason, the same plate of mica, which 
exhibits bands in the spectrum formed by a prism, may also exhibit bands in the spec- 
trum formed by a grating upon a lens or by reflection from a striated surface, though 
the proportionate degree of separation of the colours, in different parts of the spec- 
trum, is exceedingly different in these cases. 
The intervals of the bands will, however, always be approximately determined by 
certain numerical changes in the value of R, and there will, therefore, always be nearly 
the same number of bands on the spectrum, and this number will always be nearly 
the same as the number of bands remarked by Mr. Talbot when the spectrum is 
seen distinctly. The bands, therefore, will generally become broader as the spec- 
trum becomes broader, that is, as the eye (supposed at first too far off for distinct 
vision) approaches to the position of distinct vision. It may, however, happen that, 
as a becomes small, the changes in the value of s will not nearly correspond to those 
of R, and therefore, in a position intermediate to that at which these bands and that 
at which Mr. Talbot’s bands are clearly seen, no bands whatever may be visible. 
In the whole of the special conclusions deduced from the theory, the agreement 
with observation is complete. I wish it to be clearly understood that I confine 
this statement to the general appearance of the phenomena, for measures are yet 
