VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 1Q1 



that if the rays of light, after being variously bent towards the focus, were no 

 longer to move with the same common velocity, the image formed at the focus 

 of Dr. Bradley's telescope, would be elongated in the direction of the aberra- 

 tion. Those who have attended to this subject will be at no loss in discerning 

 the reason of this. The extent of that lengthened image would depend on the 

 difference of velocity which would obtain among the converging rays, and would 

 probably increase according to the magnitude of the aperture of the object-glass. 

 But such a phenomenon being contrary to experience, it follows, that the un- 

 equal bending of the rays does not give them unequal velocities, while moving 

 in the same medium. This is another property with regard to the motion of 

 light which may be considered as proved experimentally by Dr. Bradley's obser- 

 vations, and which doubtless would have occurred to him if he had had occasion 

 to trace the refraction of a pencil of parallel rays at the object-glass of his 

 telescope. 



To conclude : in bringing this question concerning the velocity of light to the 

 issue of an experiment, that fluid would doubtless be most proper for the teles- 

 cope which absorbs the fewest rays, and possesses the greatest refractive density, 

 and which at the same time is not liable to generate air-bubbles. To compensate 

 for the unavoidable loss of light, which by Mr. Canton's and Dr. Priestley's ex- 

 periments is found to be considerable in such cases, it perhaps may be necessary 

 to use an achromatic object-glass for the sake of a large aperture, and of such a 

 figure as to shorten the focal distance as much as the observations of such a small 

 angle can admit of. Some contrivance too will be requisite to keep the whole 

 space between the object-glass and the eye-glass always full, notwithstanding the 

 expansions and contractions of the confined fluid by heat and cold, or its waste 

 by evaporation. 



It might prove a very considerable abridgment of the necessary apparatus, if 

 this kind of telescope could be connected with the common telescope of a mural 

 quadrant, or zenith sector, and their axes made perfectly parallel by previous ob- 

 servations of a proper terrestrial object. But as there would be some room for 

 apprehending that the exact adjustment of the axes might be affected in raising 

 the telescopes afterwards for celestial observations, this might be examined into 

 by directing them to some star situated in, or very near, the ecliptic, and taking 

 its meridian altitudes at a time of the year when it is in quadrature with the sun, 

 in which case it would have no aberration. But either in this way, or with two 

 separate instruments, the experiment miqjht be made in a few nights, by taking 

 the zenith distance of a proper star, the plane of the instruments being alter- 

 nately turned different ways in observing, to get the true zenith distance inde- 

 pendent of the error of the line of collimation ; or the meridian altitude of the 

 pole star may be observed in December above and below the pole, which will 



