77 
taken on a scale of the sun’s surface of about three feet, but 
much larger as I show it on the screen, we have the true 
appearance of the sun’s luminous envelope produced by its 
own light, and shown here in black and white entirely by 
means of photography. With this photograph before him, 
the most skilful wood engraver would find it difficult to 
give a truthful reproduction of the picture. I believe it to 
be impossible by any other than a photographic method to 
show what is here seen. 
I also show a photographic copy of two photographs of 
parts of the sun’s surface, including a group of spots taken 
in 1878 — one at 6h. 47m., and the other at 7h. 37m. In the 
50 minutes interval, it will be seen that changes have taken 
place distinctly observable in the photographs, but they 
would probably have altogether escaped eye observation. 
Before leaving this subject I wish to refer to a very re- 
markable_ passage in a paper read before the Academy of 
Sciences in Paris, on the 11th January, 1886, by M. Janssen, 
in which he says : — “ In a note presented to the Academy on 
December 31st, 1877, in the notice inserted in the Annucdre 
du Bureau des Longitudes, for the year 1879, and in the 
opening discourse of the Congress of the French Association 
for the advancement of Science, held at Rochelle in 1882, 
I said that photography offered, not only as is generally 
believed, the means of fixing luminous images, but that it 
constitutes a method of discovery in the sciences, and 
especially in astronomy. I added that the sensitive film of 
the photographic plate, by reason of its admirable property 
of giving us fixed images, of forming them with a collection 
of rays much more extended than those which affect our 
retina, and then of permitting the accumulation of radiations 
during a time, as one may say, unlimited ; that this sensitive 
film, I said, ought to be considered as the true retina of the 
scientist;” — and while exhibiting to you specimens of the 
very beautiful work of Messrs. Paul and Prosper Henry of 
the Paris Observatory, I wish to call attention to the fact 
