68 Oqyfain W. de W. Ahneij [Feb. 25, 



I have diagrammatically shown the amount of different colours 

 which penetrated on the same days, taking the Eiffel as ten. It will 

 be seen that on December 23 we have really very little violet and less 

 than half the green, although we have four-fifths of the red. 



The next diagram before you shows the minimum loss of light 

 which I have observed for different air thicknesses. On the top we 



Fig. 3. 



^AlUtude) 8o'o of A Se/i tys. '^J\roo, 



Tlb^" 



Proportions of Transmitted Colours. 



have the calculated intensities of the different rays outside our 

 atmosphere. Thus wc have that through one atmosphere, and two, 

 three, and four ; and you will see what enormous absorption there is 

 in the blue end at four atmospheres. The areas of these curves, 

 which give the total luminosity of the light, are 761, 662, 577, 503, 

 and 439 ; and if observed as astronomers observe the absorption of 

 light, by means of stellar observations, they would have had the values, 

 761, 664, 578, 504, and 439 — a very close approximation one to the 

 other. 



Next notice in the diagram that the top of the curve gradually 

 inclines to go to the red end of the spectrum as you get the light 

 transmitted through more and more air, and I should like to show 

 you that this is the case in a laboratory experiment. Taking a slide 

 with a wide and long slot in it, a portion is occupied by a right- 

 angled prism, one of the angles of 45° being towards the centre of 

 the slot. By sliding this prism in front of the spectrum I can deflect 

 outwards any portion of the spectrum I like, and by a mirror can 

 reflect it through a second lens, forming a patch of light on the screen 

 overlapping the patch of light formed by the undeflected rays. If the 

 two patches be^xactly equal, white light is formed. Now, by placing 

 a rod as before in front of the patch, I have two coloured stripes in a 

 white field, and though the background remains of the same intensity 

 of white, the intensities of the two stripes can be altered by moving 



