48 _ Obfervations on Platina, 
pofed of arfenic, red copper, and tin, the proportions ot 
which they vary to a certain degree. This metal, which is 
attacked by all acids, foon tarnifhes when expofed to the 
air. To preferve the polifh neceflary to thefe fpecula, re- 
quires a care and attention which few people can. bettew ; 
and the great fuperiority of reflecting over refraéting tele- 
{copes has not induced aftronomers to give them the pre- 
ference, as itis exceedingly difficult to guard them from ruft, 
efpecially when they are frequently ufed in the night-time. 
' Herfchel, however, did not entertain the fame apprehen- 
fions; and by means of a large refleSting telefeope, that 
celebrated aftronomer was enabled to difcover the planet 
which bears his name. With this inftrument alfo he made 
other interefting difcoveries. Herfchel, we are aflured, em- 
ploys magnifying powers which aftonifh all aftronomers ; 
but when they fhall employ the like means to examine the 
ftars, their aftonifhment may perhaps be leflened. We are 
indeed taught by experience, that the image of an objeét is 
never very diftinct, unlefs it oceupy on the retina the fpace 
of half a minute; and we may affure ourfelves of this, by 
looking at Jupiter through a good refleéting or refracting 
telefeope, which magnifies the diameters of objects more 
than a hundred times. The difk of Jupiter, which will be 
well defined on being magnified to this degree, ceafes to be 
fo when the magnifying power of the telefcope is reduced to 
four or five times. This fact is confirmed by a feries of exs 
periments made at Calais by my colleague Fourcroy. It is 
then evident, that extraordinary magnifying powers are more 
favourable than hurtful in obfervations made on objeéts the 
diameters of which are infinitely fmall. 
If the difcoveries of Herfchel were not fufficient to fhew 
the great fuperiority of refle€ting over refraCting telefcopes, 
it would not be difficult to call in theory to the aid of expe- 
rience; and it might be proved, that the aberration of re- 
frangibility, which cannot take place in reflectors, is never 
entirely remoyed in the beft achromatic tclefcopes, The 
laws 
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