30 MR SWAN ON THE CONSTITUTION OF FLAME. 
or 180°—z would be least, and consequently 6 would be greatest, for the most re- 
frangible rays: and the spectrum would appear wider towards the more refracted 
end. 
This may possibly explain the difference of breadth in the more refracted 
regions, as actually observed by Professor DraPpER—a difference confessedly small 
—even although irradiation, by increasing the apparent breadth of the brighter 
and less refrangible regions of the spectrum, must have produced a slightly com- 
pensating effect. 
I must not omit referring here to one of Professor DRApER’s observations, 
which I have completely verified, namely, the greatly superior breadth of the 
yellow line R of Fraunnorer, when observed by his method, compared with the 
rest of the spectrum. The reason he assigns for this striking phenomenon is un- 
questionably correct, namely, that the homogeneous yellow light of a flame pro- 
ceeds from its envelope. It must not be supposed, however, that this fact affords 
the slightest evidence in favour of his peculiar views regarding the constitution of 
flame ; for I have elsewhere shown,* that the envelope of a flame produces light 
of all degrees of refrangibility between the limits (nearly) of the lines D and G of 
FRAUNHOFER, while rays of precisely the same degrees of refrangibility are pro- 
duced by the bright interior conoid of the flame. 
I will now advert to an objection which I can imagine may be made to the 
conclusion I would derive from my observations. It may be argued, that if such 
a. connexion exist between the temperature of combustion, and the refrangibility 
of the resulting light, as Professor DraPErR assumes, and if he be correct in 
thinking that the temperature of a flame increases from the interior to the 
exterior, then there must be a series of strata of different colours, even although ob- 
servation may fail in detecting them. To this I would reply, that the instrumental 
means I have employed were certainly capable of detecting any such structure, 
provided its thickness was not much less than ‘005 inch; and if we suppose it 
confined to such narrow limits, it must cease to be regarded as the general struc- 
ture of the flame, whose walls greatly exceed such a thickness. If, moreover, we 
adopt the notion of such an extremely thin stratum of varying colour, it will be 
difficult to imagine so great a variation of temperature from point to point of its 
very small thickness as will be necessary to produce the corresponding diversity 
of colour, which is supposed to exist. 
Having now shown that a careful series of experiments have failed to afford 
any evidence in favour of Professor Drarrr’s theory of the constitution of flame, 
I will next examine the reasoning by means of which he endeavours to demon- 
strate his views a priori. It seems to me that he makes two quite gratuitous 
assumptions. If we refer to his experiments on the “ Production of Light by 
* Edinburgh Transactions, vol. xxi., p. 71. 
