achromatic compound Microscope. 175 



til received may be enlarged without impediment, and the 

 light and distinctness greatly increased, thus constituting the 

 important advantage of combination. 



The triple object-glass is thus very superior to a double one 

 when each is used singly, and the union of two triple ones has 

 been already proved in England to be eminently effective. 

 Tulley's 0.9 inch glass singly admits a pencil of near 20°, and 

 his combination, one of 38°. 



These triple glasses and the double ones of Professor Amici 

 are adapted by the form of their curves each to its respective 

 place ; but the foreign double glasses which have their flint 

 lens plano-concave, and particularly those of Utzschneider be- 

 fore-mentioned, are made much on one model, and intended to 

 be each good alone. It might seem but reasonable to infer 

 from this, that they would be unfit to be combined : and ac- 

 cordingly when screwed together, most of the numerous prac- 

 ticable changes of his series of five glasses, of which as many 

 as four may be united for use, have much indistinctness from 

 spherical error; and this is I think the case with all those 

 combinations which the maker contemplated. Some peculia- 

 rities, however, observed two years ago in Chevalier's object- 

 glasses, led me to undertake a close examination of these, which 

 were liberally placed at my disposal for the purpose, in the 

 hope of discovering the cause for a discrepancy which appear- 

 ed in their effects. 



I found that with a part of the combinations, the image of 

 any bright point that was at some distance from the centre of 

 the field had a faint light or coma stretching outwards from it ; 

 with others the coma was as much inwards. 



The spherical aberration was in general much over- corrected, 

 but in some triple and quadruple combinations the opposite 

 error showed itself ; and out of the whole number one triple 

 and one quadruple were remarkably beautiful and distinct. 



The result of this investigation was to disclose or confirm to 

 me the existence of some properties of the double object-glass, 

 which have not I believe been hitherto recorded, and which it 

 ih now my purpose to describe only in connection with the sub- 

 ject before us, — the improvement of the microscope. 



With ibis in view J would premise, that the plano-concave 



