66 Captain W. de W. Ahney [Feb. 25, 



homogeneous fluid, though we know that mastic will not dissolve in 

 water. Now a wave-length in the red is about 1/40,000 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 super- 

 fluous 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 colour. Now, what is the cause of this change in colour ? 

 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 

 amongst 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 not sink down in an atmosphere of 

 normal density — or as vapour. When present as vapour the air is < 

 much more transparent, and it is a common expression to use, that 

 when distant hills look " so close " rain may be expected shortly to 

 follow, since the water is present in a state to precipitate in larger 

 particles ; but when present as small particles of water the hills 

 look very distant, owing to what we may call the haze between us and 

 them. In recent weeks every one has been able to see very multiplied 

 effects of such haze. The ends of long streets, for instance, have 

 been scarcely visible though the sun may have been shining, and at 

 night the long vistas of gas lamps have shown light having an in- 

 creasing redness as they became more distant. Every one admits the 



