112 



Their experi- 

 ments on it, 

 which are in 

 favour of the 

 .principle. 



To make a 

 perfect object 

 glafs, the re- 

 tractive power 

 of the interpofed 

 fluid fhould be 

 the fame with 

 that of the glafs. 



Water reduces 

 the confufion to 

 one feventh ; 



but oil, though 

 its power is 

 nearer that of 

 glafs, does not 

 reduce it fo 

 znuch. 



MEMOIR ON ACHROMATIC GLASSES. 



lating it fenfibly, fo that the difficulty of not producing Tome 

 inequalities of curvature in large glades muft be very great. 

 Perhaps it is impodible to avoid this imperfection, but it may 

 be corrected : and our colleague affirms, that the efFea of the 

 imperfections of the four interior fcrfaccs of the three glades 

 may be diminifhed confiderably, by the introduction of a dia- 

 phanous fluid between them. Experiments alone can decide 

 the fact. We took therefore an achromatic telefcope three 

 feet long, the aperture of which was about three inches. The 

 two lenfes compofing the object glafs, being about half an inch 

 diftant from each other, we introduced between them a thin 

 Bohemian glafs not wrought. In this date it is obvious the 

 telefcope mud be very bad ; and accordingly, on directing it 

 to fome writing, we were obliged to bring this within eleven 

 yards and half of it, to diftinguilh the letters. Having then 

 poured pure water between the object glades, fo as to fill the 

 intervals between them, we found the letters not more difficult 

 to be didinguiflied at the didance of fixty-two yards. We 

 might have purfued our experiments further, but this was 

 fufficient to edablilh the principle, which was all we had in 

 view." 



At this time I was ordered to Bred, which put a dop to my 

 purfuit. In my memoir I had advanced, that the confufion 

 arifing from the irregularity of the glafs of a telefcope, was to 

 the confufion which takes place when the glafs is immerfed in 

 a fluid, as the refraction of the glafs is to the difference of 

 refraction between the glafs and the fluid : confequently, if 

 the fluid have the fame refractive power as the glafs, no altera- 

 tion in the diflinctnefs of the object will be perceptible ; but if 

 the glafs be immerfed in water, the confufion will be about a 

 feventh only of what would have taken place, had the object 

 been feen directly through the irregular glafs. This calcula- 

 tion, it is true, fuppofes the glafs to be in perfect contact with 

 the fluid ; but there are obvioufly many caufes by winch this 

 contact may be influenced. It may have been owing to fome 

 fuch caufe, that oil, the refractive power of which differs leli 

 from that of glafs, was found by experiment not to anfwer fo 

 well as water; for writing, which was legible at the diflance 

 af 130 feet when water was employed, could be read only at 

 eighty-eight feet when oil was interpofed between the glaffes *. 



* On this fubjea, fee Philof. Journal, 4to, IIL 000. 

 2 By 



