18 LECTURES TO SCIENCE TE AGREES. 



right into those portions which are usually invisible. To 

 obtain this photograph and those which I shall show you 

 directly, the spectrum was thrown on the sensitive com- 

 pound after passing through ordinary glass prisms and glass 

 lenses. If I had employed quartz lenses and prisms, 

 or Iceland spar lenses and prisms, I should have been 

 able to obtain an impression much further in the ultra- 

 violet, because glass cuts off these rays to a great extent ; 

 but as photographers use glass objectives, I thought it best 

 to show you the spectra as produced through this medium. 



I now wish you to compare the spectrum when photo- 

 graphed on other silver compounds, with that already shown. 

 You will notice that the iodide shows the greatest impressi- 

 bility to those rays which correspond to G, whose wave- 

 length answers to about 4,300 metres, 1 at which point 



it seems to tumble down a precipice in the direction of 

 the violet. The lowest ray which effects it is the E, which 

 corresponds to about 5,200. The maximum effect produced on 

 the bromide seems to take place at about the same wave-length 

 and to diminish more gradually and regularly in both directions. 

 On the chloride the intensity seems to be nearly of the 

 same character as that of the iodide, though the fall in 

 effectiveness is not so marked towards the H line. We may 

 say, then, the gradients of the sensitiveness of the bromide 

 to the spectrum are far less steep than those of the iodide 

 and chloride. The question comes, then, is it not possible 

 to have some compound which shall give a gentler gra- 

 dient than the bromide, and thus enable us to photograph 

 further in both directions, or, if that be not possible, cannot 

 we change the point of maximum effect to a point nearer 

 towards the red end, by employing a different silver com- 

 pound, and yet preserve the same gradient, as the bromide ? 

 In either case we should be able to photograph further down 

 towards the A line. 



Each ray of light, as you doubtless are aware, is caused by a 

 different vibratory motion of the all- space-pervading ether. 

 The waves producing the red rays are longer than those pro- 

 ducing the orange, the orange than the green, and so on. Now 



1 In future we shall refer all these, wave-lengths to the same scale 



of metres ' 



