1816.} Dr. John Robison. — 188 
other things are given, being directly as the first, and inversely as 
the second. It is not, however, the general or the medium velocity 
with which light traverses space, but it is the particular velocity 
with which it traverses the tube of the telescope, that determines 
the quantity of this aberration. Were it possible, therefore, to 
increase or diminish that velocity, the aberration would be dimi- 
nished in the first case, and increased in the second. But according 
to the principles now generally received in optics, the velocity of 
light is increased when it traverses a denser medium, or one in 
which the refraction is greater; and therefore were the tube of a 
telescope to be filled with water instead of air, the aberration would 
be diminished. Professor Robison, and his friend Mr. Wilson, 
Professor of Astronomy at Glasgow, had speculated much on this 
subject, and made many attempts to obtain a water telescope, but 
hitherto without effect. A paper of Boscovich on the same subject 
seemed to suggest some new views, that might render the experi- 
ment more easy to be made. ‘That philosopher maintained that in 
ascertaining the effect of a water telescope on the motion of light, 
the observation of celestial objects might be dispensed with, and 
that of terrestrial substituted in its place. He argued that while 
light moves with an uniform velocity, the telescope must be di- 
rected, not to the point of space which the object occupied when 
the particle was sent off which is entering the telescope, but toa 
point advanced before it by a space just equal to that which both the 
object and the observer have passed over in the time in which the 
particle has passed from the object to the eye. It is therefore 
directed exactly to the place which the object is in when the light 
‘from it enters the eye. If, therefore, the ray, on entering the tele- 
scope, is made to move faster than it did betore, the telescope must 
net be inclined so much, and the apparent place of the object will 
fall behind its true place. If the ray is retarded on entering the 
water, the contrary must happen. Hence a number of very unex- 
pected phenomena would result, affording, without having recourse 
to the heavenly bodies, a direct proof of the motion of the earth 
in its orbit, as well as a resolution of the question whether light is_ 
accelerated or retarded on passing from a rarer to a denser 
medium.* 
On this reasoning Professor Robison has very well remarked that 
it would be just if the light, on entering the water telescope, had 
only its velocity changed, and not its direction. But this is not the 
case ; for the ray that is to go down the axis of the telescope is not 
perpendicular to the surface of the fluid; it makes an angle with it, 
depending on the aberration, and therefore in some cases less by 
20” than aright angle. On this account the effect is not produced 
which Boscovich’s reasonings lead us to expect. 
The sequel of the paper is also full of ingenious remarks, 
* Boscovich, Opera Math, tom, ii, opuse. 3. 
(To be continued, ) 
