428 PHILOSOPHICAL TRANSACTIONS. [aNNO 1753. 



the speculums, under limited apertures, be justly figured, no such circle of aber- 

 ration can confuse the image; but if the speculums are of a spherical figure, 

 with too large apertures, then a circle of aberration will take place; as it also 

 will when the figure deviates from the circular towards the hyperbolic, even 

 under a small aperture, and the same thing will happen, if the spherical figure 

 be inaccurate. 



About 3 days from the change of the moon, her whole body is visible; that 

 part of the limb, which is directly enlightened by the solar rays appearing to the 

 naked eye, as an arc of a greater circle than the other, which receives the reflex 

 light from the earth. Look through a refracting telescope, and you will per 

 ceive the apparent diiFerence of these circles very much diminished; and if they 

 be viewed with a good reflector, they will be perfectly reduced to an equality, 

 even if measured with a micrometer in the focus. 



If a reflecting telescope, well constructed, be directed any considerable time 

 to the sun, such a circle of aberration will be generated, from the little spe- 

 culums being heated, and thereby its figure altered, from the sun's rays falling 

 condensed on it from the great one; and if it continues long under this circum- 

 stance, the image will be rendered utterly indistinct and confused. 



This we were thoroughly convinced of at the above-mentioned transit of Mer- 

 cury; for a good reflector, which we used in taking, with the micrometer, the 

 differences of right ascension and declination between the planet and the sun's 

 limb, having been a good while exposed to the direct rays, was found at last to 

 give a very indistinct image; but was restored to its former degree of perfection, 

 by turning it from the sun, and screwing off" the eye-piece, so as to admit the 

 cool air into the great tube, by which the over-heated small speculum soon re- 

 ■ covered its due temper and figure. The last-mentioned effect is scarcely sensible 

 in the less reflectors of small apertures ; but in those of large ones it is very con- 

 siderable. 



Dr. Bevis, Mr. Canton, and Mr. Bird, who viewed Mercury going off the 

 sun, with very good reflectors of dififerent lengths, assured him, they saw him 

 quite distinct, and free from any corona, or circle of aberration, and the sun's 

 limb perfectly well defined. And he appeared to Mr. S. through a reflector of 4 

 feet focus, magnifying about 135 times, as truly defined as he could wish to see 

 a black circle on a white ground. On this occasion however Mr. S. takes notice, 

 that during the whole time of this transit of Mercury, the air was perfectly calm 

 with us : but that, in the last two transits of Mercury over the sun, viz. in the 

 years 1736 and 1743, both the sun's and Mercury's limbs appeared to him indis- 

 tinct, and surrounded with something like what this gentleman calls a luminous 

 crown, or circle of aberration ; though Mr. S. at both these times made use of 

 reflecting telescopes, which he had by former trials esteemed good. But it is to 



