VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 5Q5 



importance of a power to penetrate into space, I laid aside the work of the small 

 mirror, which was then in hand, and completed the instrument in its present form. 

 " Oct. 10, 17Q1, I saw the 4th satellite and the ring of Saturn, in the 40-feet 

 speculum, without an eye-glass." The magnifying power on that occasion could 

 not exceed 60 or 70 ; but the great penetrating power made full amends for the 

 lowness of the former ; notwithstanding the greatest part of it must have been 

 lost for* want of a greater opening of the iris, which could not take in the whole 

 pencil of rays, for this could not be less than 7 or 8 tenths of an inch. * 



Among other instances of the superior effects of penetration into space, I should 

 mention the discovery of an additional 6th satellite of Saturn on the 28th of Au- 

 gust, 1789 ; and of a 7th, on the 11th of September, in the same year ; which 

 were first pointed out by this instrument. It is true that both satellites are within 

 the reach of the 20-feet telescope ; but it should be remembered, that when an 

 object is once discovered by a superior power, an inferior one will suffice to see it 

 afterwards. I need not add, that neither the 7 nor 10-feet telescopes will reach 

 them ; their powers, 20 and 29, are not sufficient to penetrate to such distant 

 objects, when the brightness of them is not more than that of these satellites. It 

 is also evident, that the failure in these latter instruments, arises not from want of 

 magnifying power ; as either of them has much more than sufficient for the pur- 

 pose. Nov. 5, 179J > I viewed Saturn with the 20 and 40-feet telescopes. " 20- 

 feet : The 5th satellite of Saturn is very small. The 1st, 2d, 3d, 4th, 5th, and 

 the new 6th satellite, are in their calculated places." " 40-feet : I see the new 6th 

 satellite much better with this instrument than with the 20-feet. The 5th is also 

 much larger here than in the 20-feet ; in which it was nearly the same size as a 

 small fixed star, but here it is considerably larger than that star." Here the supe- 

 rior penetrating power of the 40-feet telescope showed itself on the 6th satellite of 

 Saturn, which is a very faint object ; as it had also a considerable advantage in 

 magnifying power, the disc of the 5th satellite appeared larger than in the 20-feet. 

 But the small star, which may be said to be beyond the reach of magnifying 

 power, could only profit by the superiority of the other power. 



Nov. 21, 1791, 40-feet reflector, ; power 370. " The black division on the 

 ring is as dark as the heavens about Saturn, and of the same colour." " The sha- 

 dow of the body of Saturn is visible on the ring, on the following side ; its colour 

 is very different from that of the dark division. The 5th satellite is less than the 

 3d : it is even less than the 2d." 20-feet reflector ; power 300. " The 3d sa- 

 tellite seems to be smaller than it was the last night but one. The 4th satellite 

 seems to be larger than it was the 19th. This telescope shows the satellites not 

 nearly so well as the 40-feet." Here the magnifying power being nearly alike, the 

 superiority of the 40-feet telescope must be ascribed to its penetrating power. 



The different nature of the two powers above-mentioned being thus evidently 

 established, I must now remark that, in some respects, they even interfere with 

 each other; a few instances of which I shall give. August 24, 1783, I viewed 



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