OF ARTS AND SCIENCES. 



173 



consequently the image is sonaewhat blurred, both from forced devel- 

 opment as well as from diffused light. The scientific value, then, of 

 these phenomena consist in enabling us to photograph an image of that 

 portion of the spectrum which comprises rays of less degree of refran- 

 gibility than line F, and especially those between lines E and C, and 

 perhaps also at B. It is in this part of the spectrum that many natural 

 colors absorb the rays of light. All these photographs of absorption 

 bands exhibited to the Academy were taken by the dried silver bromide 

 collodion colored by oeosin and fuchsin. 



We devised for these experiments a new form of heliostat, the beam 

 of sunlight reflected from which varied so slightly that we were able 

 to expose some of our plates for an hour and a half without any re- 

 adjustment of the instrument. The cheap cost of this form of heliostat 

 and its simple mechanism, compared with other instruments of equal 

 accuracy, induce me to present its description to the Academy. 



As will readily be seen, this instrument consists of a stand, equa- 

 torially mounted, and containing a small watch movement, in the train 

 of which a wheel is connected, which revolves once in twenty-four 

 hours. In the centre of this wheel a pinion is fixed, upon which a 

 mirror can be adjusted and firmly set. When placed in its correct 

 position, the reflection of a central beam of sunlight is thrown directly 

 in a line C D parallel to the polar axis of the earth. An upright 

 rod, carrying another silvered mirror B, intercepts at any angle to the 

 first mirror this central beam of light, and reflects it in any desired 

 position. In our experiments, the path of this beam is horizontjil, and 

 passes through a hole in a dark shutter upon the slit of the spectro- 



