and its Utility m the Arts. 23 
taws of reflestion are invariable, but thofe of refraGtion and 
difperfion are fufceptible of variations, The fubftance of 
glafs is never perfectly homogeneous. Reflection, optical 
illufions, and the lofs of light, are unavoidable in achromatic 
telefcopes, on account of the multiplicity of glaffes em- 
ployed in their conftruétion; but the fpecula of telefcopes, 
whatever be the nature of the fubftance of which they are 
compofed, are not liable to the fame faults. 
In fpeaking of achromatic telefcopes, it is impoffible not 
te recollect, that the immortal Euler was led to the difcovery 
of them by a reafoning purely metaphyfical. By reflecting 
on the ftructure of the eye, this great geometrician con- 
eluded, that the Supreme Being had compofed that organ 
of different humours, only for the purpofe of deftroying the 
aberration of refrangibility. If the metaphyfical ideas of 
Euler on the firu€ture of the eye do not accord with the 
numerous experiments which my colleacue Tenon enabled 
me to make on the humours which compofe that organ; 
the inference he drew from them was not lefs happy, fince 
it fuggefted to him the plan of making achromatic telefcopes 
by employing two different refracting fubftances. The 
Enelifh exclaimed at firft againft this idea of Euler; but 
John Doillond, who at firft oppoied that eminent mathema- 
tician’s theory, found in the great difperfion of flint glafs 
the means of realifing it. The nature of the refearches of 
that genious optician, and the advantages he derived from 
-them, have well entitled him to fhare with Euler in the 
glory of this noble and important difcovery. 
The glafs of lead is not the only kind that caufes a ftrong 
difperfion; for feveral other kinds of metallic glafs poffels 
the fame ufeful property. Some philofophers have imagined 
that there is a conftant relation between the fpecific gravity 
of glafs and its difperfion; but it is to be remarked, that 
the difperfion which takes place in ether and {pirit of wine 
is fironger than that produced in water, which is a much 
heavier liquor, Thovgh opticians hitherto have employed 
C4 only 
