ELISHA MITCHELL SCIENTIFIC SOCIETY. 33 



We must, then, to decide this matter of temperature, have 

 recurrence to auxiliary appliances, these recording their results 

 throuo-h the sense of si^ht. As achievements in this line we 

 have a mercurial thermometer, reading to the one-thousandth 

 part of a degree, the exceedingly delicate bolometer of Langley, 

 and lastly Vernon Boys' modification of D'ArsonvaPs instru- 

 ment, which he calls the Radiomicrometer. This last measures 

 the one luindrcd-millionth of a degree. In the report of its 

 exhibition before the Royal Society it is said to show the heat cast 

 by a candle-flame on a half-penny piece 1,168 feet distant. This 

 gives the present limit to our process of determining tempera- 

 ture, and there is no reason for saying that this limit may not at 

 some time be transcended. 



The sense of hearing seems to reach its limits rather quickly. 

 As, accord i no; to the theory of sound, heariu": is but the result of 

 the beating of sound-waves upon apparatus placed within the 

 ear, it does not lie far to infer that the bounds of hearing will 

 be determined by the sensibility to the greater or less rapidity 

 of the strokes. It is found that sounds with less than sixteen 

 beats to the second and those with more than thirty-five thousand 

 do not impress themselves upon the average ear. A wide scope, 

 but still a decided limit, and we can let the fancy play at will 

 among the "voiceless" sounds which must fill the world around 

 us. 



When we consider the sense of sight we find assistance from 

 magnifying instruments very essential. The limit of the unaided 

 sight is soon reached. If a line an inch long be cut into ten 

 equal parts we can easily see each separate one. If divided into 

 one hundred such parts they merge into one another, and it 

 requires a clear vision to separate them. This separation is 

 easily accomplished by the microscope, however. Yet when each 

 one of these divisions is further divided into one thousand, or 

 the whole line into one hundred thousand, even the microscope 

 and light itself fails us, for at this point the waves of the light- 

 rays interfere witli one another and the divisions, though sfiarp- 

 cut, seem blui-red. So that the very force of nature which affects 

 the organ of sight itself sets a limit to its usefulness. 



