Sroney—Compressed Air Support for Specula. 267 
the lens sags down, owing to its own weight, and assumes a very 
sensibly different shape from what it has when the telescope is 
pointed horizontally. In fact this distortion of form, although 
considerable, does not sensibly affect the image. That this can 
be the case may be easily seen by 
remembering that the waves of light m 
which constitute the rays a, b, ¢, d, €o 
must reach the point o in exactly the 
same phase, or very nearly the same Haat 
phase, in order that they may be 
able to unite and form an image there. Now this condition 
will be fulfilled if these rays all take the same time to travel 
from a wave-front mn outside the lens, to o. Hach of the 
rays passes first through air, then through the lens, and then 
through air. Light travels slower in glass than in air; and accord- 
ingly the requisite condition will be fulfilled if the form of the lens 
obliges those rays whose course is less bent to pass through a duly 
regulated greater thickness of glass. The waves of the central ray 
have the shortest distance to travel, but find the thickest part of 
the lens in their path. They are therefore most retarded. The 
rays } and d have farther to travel, but meet with a thinner part 
of the lens and are less retarded, while a and e which have the 
longest journey are still less retarded; and if the lens has been 
properly figured they all reach o at the same instant of time. 
Now let the telescope be pointed upwards. The lens, as it is 
supported only by its rim, sags down in the middle by its own 
weight. Hach ray, except those at the margin, has now a little 
farther to travel in air from mn to the lens, but it has.a little less to 
travel from the lens on to 0; and on the whole the time spent upon 
the part of its journey which is in air, is almost exactly the same 
as before. Again, the glass, though it sags down a little, has not 
become anywhere sensibly thicker or thinner. Hach ray traverses 
almost exactly the same thickness of glass as before, and occupies 
the same time in doingso. Accordingly, the air part and the glass 
part of the journey of each ray occupying almost exactly the same 
time as before, the whole time spent by each ray in travelling from 
mn to o is almost exactly the same, whether the telescope is pointed 
horizontally or vertically. 
The case of the speculum is very different. Here the journeys 
