1874.] Notices of Books. IOI 
indeed proves, that if you cut off all the heat rays, the luminous 
rays possess no warmth; while, if you cut off all the luminous 
rays, the heat rays exercise great power; but it does not prove 
to us that luminous heat plays no part in the phenomena of 
Nature. A word or two of amplification, which considerably 
simplify this part of the lecture. 
The sixth and final lecture is devoted to the important and 
rapidly growing subject of Spectrum Analysis, to which we need 
not very specially refer. This lecture is terminated by no less 
than nineteen pages of summary and conclusion. From this we 
may with advantage note here and there, of necessity somewhat 
desultorily, a remark or a generalisation. Dr. Tyndall speaks of 
Newton’s emission theory in the following terms: ‘ For a 
century it stood like a dam across the course of discovery ; but, 
like all barriers that rest upon authority and not upon truth, the 
pressure from behind increased, and swept the barrier away.” 
Of the undulatory theory, he says, ‘It had been enunciated by 
Hooke, it had been applied by Huyghens, it had been defended 
By euler. But they made no impression. ... . . It first 
took the form of a demonstrated verity in the hands of Thomas 
Young. . . . After him came Fresnel, whose transcendent 
mathematical abilities enabled him to give the theory a generality 
unattained by Young. He grasped the theory in its entirety ; 
followed the ether into the hearts of crystals of the most com- 
plicated structure, and into bodies subjected to strain and 
pressure. He showed that the facts discovered by Malus, 
Arago, Brewster, and Biot, were so many ganglia, so to speak, 
of his theoretic organism, deriving from it sustenance and 
explanation.” 
In his concluding remarks, Dr. Tyndall has addressed to the 
Americans some admirable remarks concerning the so-called 
practical scientific man and the original investigator. He has 
begged them not to confound the two, not, as is too often the 
case, to attribute the discoveries of the original worker to the 
man who applies them to the practical good of mankind. And 
he has besought them to endeavour to foster and cultivate 
original research. ‘‘ Your most difficult problem will be not to 
build institutions, but to discover men. You may erect labora- 
tories and endow them, you may furnish them with all the 
appliances needed for enquiry; in so doing you are but creating 
opportunity for the exercise of powers which come from sources 
entirely beyond yourreach. You cannot create genius by bidding 
forit. . . . . . You have scientific genius amongst you— 
not sown broadcast, believe me, it is sown thus nowhere—but 
still scattered here and there. Take all unnecessary impediments 
out of its way. Keep your sympathetic eye upon the originator 
of knowledge. Give him the freedom necessary for his researches, 
not overloading him either with the duties of tuition or of 
administration, not demanding from him so-called practical 
