334 
ME. WAEEEN DE LA ETJE ON THE 
type process, because the collodion process was far more sensitive and convenient, I 
chose the latter as best suited to my purpose, although I knew perfectly well, from 
experience, how frequently the collodion-film is rendered defective by specks, streaks, 
and even minute holes. It was open to me to employ an achromatic or a reflecting tele- 
scope of ordinary construction, and to place the sensitive plate in the principal focus ; but 
I was aware that the largest telescope I could possibly take with me would only give an 
image of a very moderate size, and that any of the before-named defects in the collodion 
might fall over and obliterate, or so confuse the impression of any prominence in one 
photograph, as to render its identification with its impression in a subsequent photograph 
a matter of impossibility. These considerations led me to think that it would be very 
desirable to employ the Kew photo-heliograph, because in this instrument the primary' 
focal image of the sun is enlarged from about half an inch in diameter to nearly 4 inches, 
which is a scale amply sufficient to counterbalance the disadvantages of the collodion 
process; but, on the other hand, the light is thus attenuated sixty-four times, besides be ing 
absorbed to some extent in passing through the two lenses composing the secondaiy mag- 
nifier, an ordinary Huyghenian eyepiece ; and this question consequently presented itself. 
Would it be possible with such an enfeebled image to get even a single impression dm'ing 
the whole duration of the totality 1 This was an extremely doubtful matter. By employ- 
ing the Kew heliograph one would evidently run the risk of returning without any pic- 
tures of the totality^ however many might be procured of the other phases of the eclipse*. 
At the meetings of the Astronomical Society, and on other occasions, I made inquhies 
of those astronomers who had witnessed the eclipse of 1851, respecting the intensity 
of the light of the corona and red flames, as compared with that of the moon, and the 
relative brightness of the one to the other ; but their answers did not tend to increase 
my hopes in respect of the possibility of procuring photographs of the totality by 
means of the Kew instrument. The general impression I formed from the information 
thus derived was, that the light emitted by the corona and red flames, taken together, 
was about equal to that of a full moon — less rather than greater; but no one recol- 
lected precisely the brightness of the prominences as compared with that of the corona. 
With this imperfect information as a guide, an attempt was made at Kew to photo- 
graph the moon, but not the slightest impression could be procured of our satellite 
by an exposure of the sensitive plate, during one whole minute f, to its image in the 
heliograph. My expectation of success in getting pictures of the totality was not great 
after this trial ; nevertheless I still thought it desirable to carry on the experiment to 
the end, on account of the value of the results if I should fortunately succeed. It 
occurred to me several times to fit up also a photographic apparatus to an achromatic 
telescope, but I finally concluded that to attempt too many things would be sure to 
result in complete failure. I endeavoured, however, to stimulate other astronomers to 
* Eeport on Celestial Photography, by the author, in the Eeports of British Association, 1859, p. 152. 
t While this paper was passing through the press a very faint impression of the moon was procui’ed nuth 
the Kew heliograph in three minutes, with chemicals which gave a very strong iuipression of it in four 
seconds in the focus of my reflector of 13 inches aperture and 10 feet focal length. — August 1862. 
