34 JOURNAL OF THE 



Just as the ear caD adapt Itself only to sounds having a lim- 

 ited range of beats or wave-lengths, so the eye can receive impres- 

 sions from those rays only which move within a limited range of 

 velocity. Beyond the red rays of the solar spectrum are many, 

 quite invisible to the eye because of the length of their waves 

 and the slowness of their movement, yet readily detected by a 

 thermometer or thermopile. And so beyond the blue are very 

 many invisible rays, too rapid and short in their motion, which 

 are revealed to us by means of sensitized paper and which can 

 only be seen then in their photographs. There are rays of light, 

 then, of which tiie eye takes no cognizance which have an impor- 

 tant warming and life-giving part to play in nature. We are 

 forced to recur to the thermometer and that valuable coadjutor 

 in scientific research, the photographic apparatus, to detect and 

 measure these for us. If we retain the old definition of lio-ht as 

 the force of which we are sensible by means of the organs of 

 sight, can we call those light-rays which do not affect the optic 

 nerves, and if not light, what are they ? 



Astronomers tell us that imperfections of instruments, impuri- 

 ties and peculiarities of the atmosphere, limit the usefulness and 

 the power of telescopes. Apparently not much more is hoped 

 from the expensive and laborious enlarging of the glass lenses. 

 No wonderful revelation is looked forward to. The limit in tin's 

 line seems nearly reached. 



In the spectroscope we have a wonderful and powerful instru- 

 ment, and one from which much has been expected. One of the 

 greatest workers with the spectroscope. Dr. Crookes, whose pains- 

 taking patience seems almost marvellous, tells us that its vision 

 extends almost to infinitesimal particles. Of sodium, for instance, 

 we can detect the one fourteen hundred-thousandth part of a 

 milligramme. 



One would hardly suspect that the sense of smell was more 

 delicate than that of sij^ht, vet the researches of Fischer and 

 Penzoldt have shown that the nose is capable of detecting some 

 substances with a delicacy two hundred and fifty times greater 

 than the spectroscope. That is, without any external assistance, 



