84 Transactions of the 



the sun through a solution of iodine in bisulphide of carbon, the 

 luminous rays will be dissolved ; and by lenses properly placed, 

 we can burn a piece of paper with the invisible or obscure rays. 

 The heat of these rays can also be well shown by the thermo- 

 electric pile. If we pass this delicate instrument through the 

 various colours of the spectrum, we see that, whilst the most in- 

 tense light is in the yellow, the greatest heat is in the red, and 

 that tliere is very little of either in the blue or violet. But these 

 rays have chemically active properties, — that is to say, they have 

 the power of decomposing, for example, silver salts. This is a 

 most important fact in photography. It can easily be shown that 

 the red rays have no chemical power, while the blue and violet 

 have a great amount, by placing two sensitized plates, one in the 

 violet and the other in the red rays. The former will be acted 

 upon, but not the latter. The same phenomenon can be seen 

 by the following experiment : — Place an electric light in a lantern 

 with four coloured glass sides, and hold in front of the red side a 

 bulb containing hydrogen and chlorine in equal proportions, the 

 two gases will combine with a flash (forming hydrochloric acid) 

 owing to the heat ;* but if we place another bulb in front of the 

 blue side, the two gases will unite with such force that the glass bulb 

 will be shattered. It has been a very interesting work to physi- 

 cists to find out by experiments the extent of chemically active 

 solar rays in different parts of the globe, and very curious results 

 have been obtained. It has been asserted that it requires a longer 

 time to fix an image in photography under a burning tropical sun, 

 than in the gloomier streets of London ; and that from twenty 

 minutes to half-an-hour was required in Mexico, where the light 

 is very intense, to produce photographic effects which in England 

 occupied a minute. Observations made at Para have shown what 

 sudden changes take place in a very short time in the power of 

 the chemical rays. 



Having learned so much about the properties of the different 

 rays, let us examine the construction of the spectroscope. There 

 are various forms of this instrument, but it will be necessary to 

 describe only a moderately simple one, as made by Steinheil of 

 Munich. f It consists of a prism (a) fixed upon a strong iron 

 stand, and a tube (h) carrying the slit [d), through which the rays 

 from the coloured flames (e and el) fall upon the prism, being 

 rendered parallel by passing through a lens. The light having 

 been refracted, is received by the telescope (/), and the image 



* The description here given of the action of light on a mixture of hydrogen 

 and chlorine is incorrect, as no action takes place when the actinic rays are 

 stopped by red glass. — Note by the Examiner. 



+ There is a figure of the instrument here described in Eoscoe's Spectrum 

 Analysis, and the letters refer to it. 



