VOL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. 42/ 



therefore, according to Mr. Short's computation, magnified about 130 times. 

 He was placed in the most commodious situation for observing the egress; his 

 smoked glass was fixed perpendicular to the axis of his telescope within a close 

 tube;, and he always used the same part of this glass. 



He took, notice, that the interior contact of Mercury's and the sun's limbs, 

 at lO'' 18'" 41% was very rapid, having observed it with a green-coloured glass 

 held over the smoked glass: immediately after which, looking through the 

 smoked glass only, he perceived that a small thread of light was still visible be- 

 tween the limbs, before what he calls the second contact took place, which was 

 not till 4 seconds after; that the exterior contact appeared stationary, or seemed 

 to last 6 or 7 seconds; that having observed the total egress with the coloured 

 glass on the smoked one, he brought Mercury on the sun's limb again, by re- 

 moving the coloured glass; and that the second total egress did not happen till 

 6 or 7 seconds after the first. When he observed him at the distance of about 

 3 of his diameters from the sun's limb with both the glasses, he remarked that 

 the same distance seemed diminished, and Mercury's diameter increased. That 

 the part of the sim's limb where Mercury went off, to the extent of 6 degrees 

 of circumference, seemed under much the same configuration, as the illumi- 

 nated limb of the moon about the quadrature, somewhat uneven and undulating. 

 The same looked also redder than the rest of the disk. This was about 18 or 

 20 seconds before Mercury disappeared, and was seen through the smoked glass 

 alone; for when the green glass was applied, the appearance in a manner vanished. 



The evening before the transit he viewed the sun with different coloured 

 glasses, variously combined with each other, and with a smoked glass; and found, 

 that a green glass before the smoked one did best; the sun appearing of a silvery 

 hue, like the moon, and the spots and the limb exceedingly well defined. 



M. de Barros, having thus described the particular phenomena, ingeniously 

 attempts to account for them all, from this single supposition ; that the disk of 

 the sun, and of Mercury seen on it, are environed with a certain corona of light 

 (like that which Sir Isaac Newton calls the circle of aberration or dissipation in 

 refracting telescopes) by which the apparent diameter of the sun is enlarged, and 

 that of Mercury contracted. But as this gentlemen made use of a reflecting 

 telescope, and as no such circle, from the known principle of reflection, can 

 take place in such a telescope, if well made, as Sir Isaac has proved long since; 

 Mr. S. thinks it not worth while to pursue him through all his particular supposi- 

 tions; but only to show that his hypothesis has really no foundation. 



Sir Isaac, as before hinted, remarks, that the images of all objects seen in 

 refracting telescopes, are surroundeil with a circle of aberration ; which is always 

 less, the longer the telescopes are. In his optics, tn avoid the indistinctness 

 arising from this circle, he would propose catadioptric telescopes, in which, if 



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