lx 



ing far more heating power than any of those revealed to us by sight. The 

 temperature was then tried at the other end of the spectrum, beyond the 

 violet, but here there was scarcely a trace of heat to be found. But on 

 placing certain substances beyond this end of the spectrum, what was 

 equally as startling as the existence of the obscure rays of heat beyond 

 the red, was the discovery that there were invisible rays beyond the violet, 

 which possessed remarkable chemical power. These are called the 

 actinic, or chemical rays, and are those which are most valuable to the 

 photographer. 



It must be remembered that all illuminated bodies possess the power 

 of reflecting the rays of light. By illuminated bodies are meant all white 

 and colored objects revealed to us by sight; as it is only by the rays of 

 light which fall upon them, being reflected from their surfaces, through 

 the eye, upon the delicate nerve tissues of the retina, that their presence 

 is revealed to us. All objects which do not reflect light are black. In 

 comprehending the art of the photographer it is important to remember 

 these facts. Hence it matters not whether certain chemical solutions are 

 exposed in the direct sunlight to these invisible rays, or whether these 

 rays are reflected from the face of a human being, a house, the moon, or 

 other illuminated object upon such solutions. Their action will work the 

 same chemical changes. These chemical rays, reflected with the others 

 from illuminated objects placed before the camera of the photographer, 

 produce an atomic change upon the sensitive solutions with which his 

 plates are prepared, and thus form upon them images of such objects as 

 are in the field of the lens at the time. 



The colored rays of the spectrum possess chemical power also, but. like 

 their heating power, it is less than that possessed by the invisible rays. 



The phenomena exhibited by the invisible heat rays at the red end of 

 the spectrum are no less remarkable than those manifested by the chemi- 

 cal rays. A correct idea, however, of the cause of these phenomena can- 

 not be well comprehended without some knowledge of the Undulatory 

 theory of Light, now almost universally accepted as the only one by 

 which all of its various phenomena are believed to be explained. 



The analogies in the phenomena of light and sound are so numerous 

 that a brief explanation of some of those of sound will enable us to com- 

 prehend more easily those of light. Sound and light are reflected in the 

 same manner, the angles of incidence and reflection being equal. Sound, 

 like light, is refracted when passing through media of different densi- 

 ties ; each may be doubled in intensity, or destroyed, by interference; 

 and each is propagated by the undulations or vibrations of the conducting 

 medium. 



The phenomena of heat and light are likewise so closely allied that a 

 theory which is applicable to one will probably explain every phenomenon 

 of the other. Heat is reflected, refracted, transmitted and polarized in the 

 same manner as light. 



All sonorous bodies create sound by imparting their vibrations to the 

 air when they are themselves thrown into vibration. When they vibrate 



