Bi 
F 
’ 
‘ 
Histological Preparations by Sunlight. 263 
focus of heat, as well as of light, is produced, which is damag- 
ing to the preparation as well as to the balsam cement of the 
objectives used. If, however, the rays from the first lens are 
permitted to come to a focus and to begin to diverge before 
striking the second, this latter can readily be adjusted so as to 
bring the illuminating rays to a handsome focus, while the heat 
tays, after passing the second lens, become parallel or even 
light ; an 
3 consequently, the rays of heat, after passing rough 
this lens, will become parallel, abile the rays a light converge 
4 a second focus. I have approximately measured the heatin 
Power of the thermal rays of the second cone when rende 
in the second focus, did not reach 90° Fah., while ie 
Same time the heat at the focus of the first cone was sufficien 
d ”? 
* The British Journal of Photography, Dee. 16, 1870, p. 590. 
