RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 55 
(in order to show the greatest extent to which I have observed the whitening or con- 
servative action of the less refrangible chemical rays to extend, and that only in one 
and a very peculiar case,) fig. 6. exhibits a spectrum photographically impressed on 
paper (No. 610.) originally prepared with successive washes of nitrate of lead, mu- 
riate of soda, and nitrate of silver, but kept for some time (five months), and thereby 
much impaired in point of sensibility. The same paper fresh prepared exhibits no 
such anomalous spectrum. 
136. The most singular and striking phenomenon exhibited in the thermic spectrum 
thus visibly impressed, is its want of continuity. It obviously consists of several 
distinct patches, of which «, (3 are the most conspicuous and intense, but are less 
distinctly separated, and of which when the sun is very strong and clear it is even 
difficult to trace the separation. The spot y, on the other hand, is round and well 
insulated. It begins to appear on the paper soon after the ovals a, (3 are fully formed, 
and when (3 has assumed a sharply rounded outline. The first symptom of its ap- 
pearance is the dulling of the wet and shining surface of the paper, which is speedily 
followed by the appearance of a small round white speck ; this continues to increase 
rapidly in size and whiteness, and at length assumes a definite and perfectly circular 
outline, within which the paper is entirely white. By degrees the oval (3 and this 
spot join and run together, forming a white streak deeply indented at the point of 
junction. It is not till this happens that similar symptoms begin to betray the ex- 
istence of a still more remote heat-spot Indeed, it generally requires another wash 
of alcohol before this can be fully brought into evidence. It is, however, perfectly 
uneauivocal, though very much feebler, and rather worse defined than y, with which 
also it is somewhat better connected than y with (3. 
137. The mean of a great number of measures, as precise as the nature of the ob- 
servation admits, gives for the situation of the centres or maxima of these spots, or 
their coordinates along the axis of the spectrum, reckoning as usual from the fiducial 
yellow, as follows : 
a - 17*5, 19*0, 18-5, 16*0, 19, 19,20, 18, 19, 20,20, 19, 16, 16, 16, 18'5, 18*3. Mean 
of all = - 18-2. 
f 3 - 26-0, 27-5, 29-0, 25*0, 255, 273. Mean — 267- 
y — 36-0, 35, 35, 34-5, 36, 36, 35'5, 36, 36, 36, 36, 36‘5, 37‘2, 355, 36*5, 36‘0, 
35-0, 36-2, 36 0, 355, 36'5. Mean — 357- 
h - 45, 46, 46-5, 45’0, 44*2, 44*3, 44‘5, 46, 44, 45*8, 45*5. Mean — 45T. 
The measures for a and (3 differ a good deal inter se, partly from the less definite na- 
ture of the maxima, and partly from having used in the measures a variety of different 
sorts of paper, which influence the extent and intensity of the spots a and (3 mate- 
rially. The measures of y and & are closer by reason of the definite nature of the rays 
which respectively go to form them. Nor does the nature of the paper used in any 
respect displace these two spots, though it influences to a great extent their apparent 
