The next diagram before you shows the minimum loss of light which I have observed for different air thicknesses.  On the top we have the calculated intensities of the different rays outside our atmosphere.  Thus we 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 deg. being toward the center of the slot.  By sliding this prism in front of the spectrum I can deflect outward 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 exactly equal, white light is formed.  Now, by placing a rod as before in front of the patch, I have two colored 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 the right angled prism through the spectrum.  The two stripes are now apparently equally luminous, and I see the point of equality is where the edge of the right angled prism is in the green.  Placing a narrow cell filled with our turbid medium in front of the slit, I find that the equality is disturbed, and I have to allow more of the yellow to come into the patch formed by the blue end of the spectrum, and consequently less of it in the red end.  I again establish equality.  Placing a thicker cell in front, equality is again disturbed, and I have to have less yellow still in the red half, and more in the blue half.  I now remove the cell, and the inequality of luminosity is still more glaring.  This shows, then, that the rays of maximum luminosity must travel toward the red as the thickness of the turbid medium is increased.

The observations at 8,000 feet, here recorded, were taken on September 15, at noon, and of course in latitude 46 deg. the sun could not be overhead, but had to traverse what would be almost exactly equivalent to the atmosphere at sea level.  It is much nearer the calculated intensity for no atmosphere intervening than it is for one atmosphere.  The explanation of this is easy.  The air is denser at sea level than at 8,000 feet up, and the lower stratum is more likely to hold small water particles or dust in suspension than is the higher.