72 
PROFESS OB TYNDALL ON THE OPTICAL DEPORTMENT OE THE 
solids and liquids of all kinds. The next section of the investigation will be devoted to 
these and kindred subjects; and to it I also postpone the complete examination of 
pepton, and of the remarkable experiments described by Dr. William Roberts, a small 
residue of which only I have failed to corroborate. 
Throughout the whole of this investigation I have had to congratulate myself on the 
zealous and efficient aid of my excellent assistant, Mr. John Cottrell. His intelligence 
in seizing my ideas, and his mechanical skill in realizing them, have rendered me 
admirable service. Without such aid, indeed, so much ground could not have been 
covered in the time. Mr. Cottrell’s junior colleague, Mr. Frank Valter, has also 
acquitted himself to my entire satisfaction. 
Royal Institution, 5th April, 1876. 
It gives me special pleasure to direct attention here to a paper by the Rev. W. H. 
Dallinger, for an advanced proof of which I am indebted to the courtesy of Dr. Lawson 
editor of the ‘ Popular Science Review.’ Mr. Dallinger and his colleague Dr. Drysdale 
are known to have pushed the microscope to its utmost power of performance at the 
present time. Their ‘ Researches into the Life-History of the Monads ’ are models of 
scientific thoroughness and concentration. Mr. Dallinger’s review of the present 
position of the doctrine of spontaneous generation, his remarks on Bacterial germs in 
relation to the limits of the powers of the microscope, his demonstration that the germs 
of monads survive in a medium raised to a temperature which destroys the adult, and 
that precipitated mastic particles like those mentioned in § 16 of this paper are not to 
be discerned by a magnifying-power of 15,000 diameters, constitute a most interesting 
and important communication. 
Note I. Action of Bacteria upon a Beam of Light. 
To trace the gradual growth and multiplication of the Bacteria by their action on a 
beam of light an infusion of beef was prepared on the 5th of October, placed in a glo- 
bular flask of about 50 cubic inches capacity, and put aside with its mouth open to the 
laboratory air. On the 8th, 9th, 10th, 11th, and 12th similar flasks were prepared and 
put aside in succession. On the 12th all the flasks were examined by the concentrated 
electric light. The freshest one showed the track of the beam as a richly coloured 
green cone. The green light was unaffected by a Nicol’s prism, which, however, 
quenched the ordinary scattered light and augmented the purity and vividness of the 
green. It was a case of fluorescence. In the second flask, one day old, the fluorescent 
beam was in great part masked by the scattered light ; the latter, however, could be 
partially quenched by a Nicol’s prism, the purity of the fluorescence being thus in part 
restored. Through the third flask, two days, and the fourth flask, three days old, the 
track of the beam was still discernible ; through the fifth flask, four days old, it was all 
