FREE BOOKS

Author's List




PREV.   NEXT  
|<   19   20   21   22   23   24   25   26   27   28   29   30   31   32   33   34   35   36   37   38   39   40   41   42   43  
44   45   46   47   48   49   50   51   52   53   54   55   56   57   58   59   60   >>  
e red end of the spectrum they are very much crowded, while towards the blue end they are more dispersed. Hence, if one were measuring the heating power of such a spectrum, many more rays would fall upon an equal surface of the thermo-pile at the red end than at the blue end; therefore the indications of the galvanometer would be fallacious. Before any thing definite could be known about the matter, it would plainly be necessary to work with an equal dispersion of all the rays. This was effected a few years ago by Dr. Draper of New York. He took the spectrum produced by diffraction instead of refraction, and measured that. In that way it was found that the heating power of the spectrum is equal in every part of it; and hence the pictures in treatises on physics that represent the heating power of the spectrum to be concentrated at the red end is not true save where the spectrum is irregularly produced. As for vision, the mechanical structure of the eye is such that radiant vibrations having a wave length between 1/37000 in. and 1/60000 in. can affect it, while longer or shorter wave lengths can not. Such waves we call light, but it is not at all improbable that some animals and insects have eyes adapted to either longer or shorter wave-lengths; in which case, what would be perfectly dark to us would be light to them. It is a familiar enough fact, that many animals, such as dogs, cats, rats, and mice, can see in the night. Some horses may be trusted to keep in the road in a dark night, when the driver cannot see even the horse itself. This has usually been accounted for by saying that their eyes are constructed so as to collect a greater number of luminous rays. It is much better explained by supposing their eyes to be constructed to respond to wave-lengths either greater or less than those of mankind. A ray of light, then, consists of a single line of undulations of a definite wave length, such that if it falls upon the eye it will produce sight; if it falls upon a thermo-pile it heats it by just the same quantity that another wave-length would heat it; if it falls upon matter in unstable chemical relations, it will do chemical work, depending upon the kinds of matter. A red ray is as effective for some substances as a violet ray is for others. The statement, then, so often lately made to do certain analogical work, namely, that a ray of light consists of three distinct parts, which may be separated from each other, and
PREV.   NEXT  
|<   19   20   21   22   23   24   25   26   27   28   29   30   31   32   33   34   35   36   37   38   39   40   41   42   43  
44   45   46   47   48   49   50   51   52   53   54   55   56   57   58   59   60   >>  



Top keywords:

spectrum

 

length

 
lengths
 

matter

 

heating

 

produced

 

greater

 
consists
 

longer

 

animals


constructed

 

shorter

 

chemical

 
thermo
 
definite
 

driver

 

analogical

 
separated
 

trusted

 

distinct


horses
 

accounted

 
single
 

undulations

 

relations

 

depending

 

substances

 

effective

 

unstable

 
quantity

produce

 

mankind

 

statement

 
collect
 

violet

 
supposing
 
respond
 

explained

 

number

 
luminous

effected

 
dispersion
 
plainly
 

Draper

 

refraction

 

measured

 

diffraction

 
dispersed
 
crowded
 

measuring