Artificial Halos. 45 



ARTIFICIAL HALOS. 



Eveeyone who lias used an air-pump has noticed the clouds of 

 vapour which forni in the receiver after a few strokes of the 

 piston, and which arise from the air yielding up a portion of its 

 moisture as the pressure is diminished. If these vapours are 

 viewed by light transmitted from a candle, prismatic colours will 

 appear ; but to insure a distinct and fine halo Mr. Slack recom- 

 mends the following plan : Place a large receiver on the princi- 

 pal plate of an air-pump, and a small one, holding about a 

 quarter as much as the former, on the smaller plate. Turn the 

 stopcock so that when the pumps are worked the small receiver 

 only shall be exhausted, the large one remaining full. When 

 a vacuum has been made, place a taper on one side of the large 

 receiver, and stand on the other, keeping the eye on a level 

 with its light, and suffering no other illumination in the room. 

 Now, suddenly turn the stopcock so that a portion of the air 

 from the large receiver shall rush to the exhausted smaller one. 

 At this moment a splendid halo will appear, and it is an inter- 

 esting and by no means an easy task to notice the exact order 

 in which the colours are exhibited. The average decision arrived 

 at in one set of experiments was as follows : A yellow light 

 seemed to rush from a circumference to a centre, forming a 

 luminous disk, which passed instantly to a red-orange hue, and 

 then to a brilliant emerald- green. At this point the green 

 central disk appeared to expand outwardly and take the form of 

 an external ring, the centre resuming an orange tint. The 

 changes in the phenomenon are exceedingly rapid, and their 

 duration so infinitesimal that it is impossible to note and describe 

 all the chromatic effects, among which some rich purple rings, 

 will be observed, before the luminous circles disappear. Thoss 

 who wish to perform the experiment with an air-pump that has 

 only a single plate should connect its receiver by a pipe and 

 stopcock with a larger closed vessel full of air, and then proceed 

 in the manner described. A large amount of light is injurious 

 to the results, as it overpowers the coloured rays. If the expe- 

 riment were performed on a large scale it would probably be- 

 effective in a lecture-room. 



Under ordinary circumstances there is enough moisture in 

 the air to give rise to pleasing effects ; but they will become 

 more striking if a few drops of water are sprinkled on the. 

 inner surface of the large receiver. It is also interesting to 

 notice the variations that occur if alcohol or liquid ammonia 

 be substituted for the water. In the latter case, the clouds 

 formed are denser and less evanescent. 



