7 
Electric Lights for Photo-micrography. 295 
"prints of some of the tissue-preparations thus reproduced. At 
present it is my desire to call your attention to certain impor- 
tant observations which I had the good fortune to make, while 
my experiments were in progress, and which it appears to me 
cannot fail to be of interest and service to all microscopists. 
During the last week of October and the first two weeks of 
November, I relied wholly on the sun as the source of illumina- 
tion for producing negatives. In this period, during which I 
had but two perfectly cloudless working days, and several frac- : 
tional days on whieh my work was continually interrupted b 
passing clouds, I had ample opportunity to convince myself 
that the uncertainty of the weather was a most serious hin- 
drance to the preparation of successful photographs of micro- 
Scopic objects, and I ceased to wonder that Huropean micro- 
Scopists, who are exposed to a climate even more variable than 
ur own, have not yet succeeded in placing the art of Photo- 
Micrography upon such a basis, as to make a convenient and 
habitual auxiliary in all microscopical investigations. This 
desirable end I believe I have attained; but it has been by 
resorting to artificial lights and thus making the success of the 
Process wholly independent of the weather. 
On the 12ih of November I commenced a series of experi- 
. 
or the production of the electric light I used a Duboscq's 
a8 a source of illumination in the preparation of photographs of 
ified objects, and that the pi were hence clearer and 
better defined than any ph coreg of similar objects I had 
hi seen bcindbanede by sunlight. I f 80 
it to produce negatives with much shorter exposures than are 
indispensable with the sun. = ' 
‘he magnesium light shared these qualities to a high degree, 
but I found that its best work was done when the object was 
Rot to be ified more than a thousand diameters, and that 
there were certain limitations to its use on test objects which 
_ Will be referred to in the sequel. 
