a, 
Ln 
288 THE NORTHERN MICROSCOPIST. 


They will combine and explode if you expose them to these chemical rays I 
speak of ; but they will not if you expose them to the lighting or heating rays 
alone. It is the power of the chemical rays which causes them to explode. 
Now, I believe, I am correct in saying it is the power of these non-luminous 
chemical rays which produces a photograph. Certainly it is not the visual or 
luminous rays. As an illustration of this, it is only necessary to call your 
attention to what is known as the dark room or developing chamber. Of 
course, light is the medium used to take a photograph, and I dare say you all 
know how sensitive the prepared plate is to light—that is, light from which 
actinic rays are not eliminated—so sensitive, indeed, that no sooner is the cap 
removed from the camera lens than the requisite impression may be said to be 
got. This so called dark room is not, however, dark, as light is admitted 
through a window of yellow glass, and as a matter of fact the window is often 
smaller than necessary. Now this yellow glass has the property of inter- 
cepting chemical rays, and visual rays alone pass through into the room; 
consequently the sensitive plate can be exposed, and other operations carried 
on as though in the dark. On the other hand, a photograph can be taken in the 
dark, providing these chemical rays are present. Take a line engraving, and 
place it directly in the sun's rays for a couple of hours; then take it into a dark 
room and place it face upwards at the bottom of a box; have ready a piece of 
aper wetted with a solution of some of the salts of silver (ammonia-chloride or 
iodide, or chloride) ; lay the above face to face with the engraving, close all up 
and keep dark for twenty-four hours, when you will find that sufficient chemical 
rays have been given off from the engraving to produce a copy on the prepared 
paper. What I want you to understand is, that there are conditions essential 
to a satisfactory ocular performance of an objective, which do not exist, or are 
not necessary, at all events to the same extent, to the production of a photo- 
graphic picture. The cutting off, by means of a stop, of the marginal zone of 
rays in an objective, does not, in my opinion, affect its working capabilities for 
peeane use to the same extent as its ocular performance in the microsco 
f you cut off the outer visual rays of an objective, there is a sad loss of light. 
This cannot be compensated for in the microscope, but in taking a photograph 
it simply necessitates a little longer exposure. 
That, gentlemen, may be said to constitute my thoughts upon the matter; 
but, in addition to that, I thought I would take the liberty of addressing a 
letter to Mr. Dallinger, a gentleman who is well known to have worked with 
high powers, and I thought it could not have escaped his consideration what 
would be necessary in these powers for their working capabilities. I wrote 
him a short, courteous note, asking him would he oblige by expressing an 
opinion on the matter. I did not hold out any leading questions at all. I am 
sorry I did not keep a copy of my letter, but it consisted of about ten lines, 
saying this matter was coming up for consideration in consequence of Abbé’s 
paper. After a week’s interval, I received this letter. It is written, I think, 
from Scarborough, where he was at the time. It seems to have been an un- 
fortunate time. I suppose all these gentlemen are away on their holidays, and 
I can imagine they don’t like to be troubled. Mr. Dallinger favoured me with 
a rather lengthy explanation. It is as follows:— 
“ AuGusT 18TH, 1882. 
**MY DEAR S1R,—I am away from home, and so placed, that writing of the 
**kind that your communication requiries is, alas, impossible. I must, there- 
‘*fore, briefly state (1) that for the last ten years I have, as a matter of actual 
seEcoees necessity, adopted the views now clearly and fully put forth by 
He goes on to qualify somewhat this statement, but it is well to remember 
that he endorses Abbé to the full. The letter continues :— 
“*(2) No one has ever appreciated or found more pleasure and profit in the 
‘fuse of the large angles with which our lenses have been more and more 
_ “perfectly provided for the last ten or twelve years than I have. As they have 

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