234 Expefimetlfs iV'th Jeht'ainattc Lenfei cemented together. 



cffuch value ihould never have been adopted in this kingdom. As it is no where faid that 

 lenfcs were conftrudled with a particular regard to the deaftty and figure of the interpofed 

 maftic ; but there feems reafon to conclude, that in every cafe an achromatic lens of the 

 ufual conftru£lion was dated to have been rendered more perfect by this treatment, — I thought' 

 it beft to overlook the theoretical objeiStions, and confine myfelf in the firil inftance to experi- 

 ment and enquiry. . For this purpofe I took an achromaticlens, confiftingof two convex lenfes 

 and one concave, of which the radii need not here be detailed. "When together they formed a 

 compound lens of 7|- inches focus, with an aperture of nearly li inch, which, upon a rough 

 trial with a magnifying power of 30 times, exhibited fcarcely any colour. . Upon filling the 

 interftices of this lens with water, the focal image became much brighter; but the focal 

 diftance was diminilhed to fomewhat lefs than fix inches, and much colour was produced 

 with as low a magnifying power as ten times. It is evident, therefore, that the lens was 

 greatly.injured by the expedient made ufe of to prevent the lofs by reflexion; and it may 

 eafily be inferred that a denfer fubftance, fuch as maftic, would have fhortened its focus and 

 difturbed its compenfations in a ftill greater degree. 



". Some time after making this experiment, I applied to the celebrated Mr. Ramfden, to whofe 

 unequalled ability and invention the arts of philofophical obfervation are fo higiily indebted. 

 I did not expe£l to receive any information tending to (hew that a combination of lenfes, 

 differing in curvature at the furfaces of appofition and achromatic in the air, could retain 

 the fame .property when either partially or totally immerfed in another tranfparent medium 

 of different refradlive or difperfive power. My principal object was, to know whether the 

 •French philofophers had conftruded leafes of fix parts duly adapted to each other ; that is to 

 fay, confidering the two portions of mafl:ic as lenfes, and not merely as a medium to prevent 

 lofs by reflexion. From him I learned that the fuppofed improvement had long fince been 

 -communicated to him by fome gentlemen of the Paris academy, who affirmea that an 

 achromatic lens of Dollond's make had been much improved by the interpofition of mafliic 

 between its parts, at the fame time that they aflerted in general terms that the fame might 

 be done with any other objedl: glafs of this kind. He put a good objedl glafs into their 

 hands for this, purpofe. The experiment was made, and the refult proved fimilar to that 

 which I have already related. The aberrations from figure as well as colour became very 

 perceptible ;— and in a word the lens, though much clearer, had loft its moft valuable 

 qualities. 



Such is the refult of a procefs which has been in the poffeffion of our learned neighbours 

 for about four-and-twenty years, and is ftill efteemed by them ; but which has never been 

 adopted by our opticians : — a procefs, the value of which, it ftiould appear, might be afcer- 

 tained in an inftant by mere infpeftion through the telefcope in queftion. We feem as if 

 driven to the fuppofition that fomething might yet remain unexplained. Has the mifchief 

 -of this procefs been overlooked by men fo acute and enlightened ? — Or has it happened that 

 the interior curvatures of the lenfes, on which they operated, were nearly parallel to each 

 other ; ,fo that the addition of light (with the magnifying power commonly ufed with an 

 •index telefcope) might be thought to offer an advantage, in angular obfervations, more than 

 ifuflicient tocompenfate for the other defedsi 



IX. De. 



