SflCH PHILOSOPHICAL TRANSACTIONS. [aNNO 1753. 



the other, when in apogaeo; so shall ms be the then distance of the solar images, 

 measuring with the micrometer, suppose, \' 10. The difference of these two 

 observations, 1 minute, is the apparent diminution of the sun's diameter. 



The little circle, whose diameter is dt, is the whole area visible at once in the 

 telescope, which is not a 3d part of the magnified diameter of the sun; but 

 since both nr at one time, and ms at another time, are visible within the teles- 

 cope's area, if good instruments are procured, Mr. S. sees no difficulty in per- 

 forming what he has proposed above, more accurately than it has ever yet been 

 done, except this one (which some time since Mr. Graham mentioned in a letter 

 to him) viz. that of defining the sun's disk, truly; and Mr. S. thinks, to do that 

 to good perfection, is beyond human art. A telescope for this use may be made 

 to magnify the sun's diameter to any degree whatever, not exceeding such degree 

 as will make any part of the line ms fall without the area of the telescope: and 

 he thinks it will be very difficult to make one with a charge so great, as not to 

 have more than a geometrical minute of the sun's apparent diameter visible 

 at once. v 



Since the sun is an object so very remote, the pencil of rays flowing from the 

 centre of its disk, and incident all over an object-lens, though it should be a 

 foot broad, would not differ sensibly from a perfect cylinder within the distance 

 of above 100 miles from its basis at the lens; though in reality the whole pencil 

 is an acute cone, whose angle at the vertex is almost evanescent. Hence it fol- 

 lows, that if the two poles of two equal object-glasses are placed at the distance, 

 suppose, of a foot from each other, the two centres c, v, of the two solar images 

 must, as to sense, remain always at that very same distance, viz. 1 foot from 

 each other, though the sun should be placed 10 times as far off as it now is; 

 but since the sun's greater distance would diminish the diameters of both of the 

 solar images; mn, added to rs, must be the true difference of the apparent dia- 

 meters of the images, and also of the sun, at different times. 



According to Mr. Azout, Harris's Lexic. Techn. vol. i, see sun, the apparent 

 diameter of the sun never exceeds 32' 45 "; hence its radius never exceeds l6' 

 12" 30'"; the tangent of which is about 476328, to the radius 100,000,000. 

 Then, as the said tangent : to the said radius :: so half an inch : to 104. 96 inches, 

 and decimal parts. According to this, if the focal length of a lens be IO4.96 

 inches and parts, it cannot collect the sun's rays to a less focus at the time of 

 his perigee, than 1 inch in diameter, or half an inch radius. 



In fig. 2, the whole circle represents a well centred object-lens, whose focal 

 length is, as above calculated, 104. 96 inches and parts, or rather a little less, 

 that the two images may be sure not to touch each other. Let the two dia- 

 meters dm, qf, divide it into 4 quadrants, but the diameter qf must be occult, or 

 delible. Let cw be half an inch, and cv equal to it. Through v, and also 



