such particles being small in dimensions compared with a wave of light.  Now, in this trough is suspended 1/1000 of a cubic inch of mastic varnish, and the water in it measures about 100 cubic inches, or is 100,000 times more in bulk than the varnish.  Under a microscope of ordinary power it is impossible to distinguish any particles of varnish; it looks like a homogeneous fluid, though we know that mastic will not dissolve in water.

Now a wave length in the red is about 1/40000 of an inch, and a little calculation will show that these particles are well within the necessary limits.  Prof.  Tyndall has delighted audiences here with an exposition of the effect of the scattering of light by small particles in the formation of artificial skies, and it would be superfluous for me to enter more into that.  Suffice it to say that when particles are small enough to form the artificial blue sky, they are fully small enough to obey the above law, and that even larger particles will suffice.  We may sum up by saying that very fine particles scatter more blue light than red light, and that consequently more red light than blue light passes through a turbid medium, and that the rays obey the law prescribed by theory.

I will exemplify this once more by using the whole spectrum and placing this cell, which contains hyposulphite of soda in solution in water, in front of the slit.  By dropping in hydrochloric acid, the sulphur separates out in minute particles; and you will see that, as the particles increase in number, the violet, blue, green, and yellow disappear one by one and only red is left, and finally the red disappears itself.

Now let me revert to the question why the sun is red at sunset.  Those who are lovers of landscape will have often seen on some bright summer’s day that the most beautiful effects are those in which the distance is almost of a match to the sky.  Distant hills, which when viewed close to are green or brown, when seen some five or ten miles away appear of a delicate and delicious, almost of a cobalt, blue color.  Now, what is the cause of this change in color?  It is simply that we have a sky formed between us and the distant ranges, the mere outline of which looms through it.  The shadows are softened so as almost to leave no trace, and we have what artists call an atmospheric effect.  If we go into another climate, such as Egypt or among the high Alps, we usually lose this effect.  Distant mountains stand out crisp with black shadows, and the want of atmosphere is much felt. [Photographs showing these differences were shown.] Let us ask to what this is due.  In such climates as England there is always a certain amount of moisture present in the atmosphere, and this moisture may be present as very minute particles of water—­so minute indeed that they will sink down in an atmosphere of normal density—­or as vapor.  When present as vapor the air is much more transparent, and it is a common expression to