820 Transactions of the Society. 



advantages of a more perfect spherical correction are inappreciable, 

 and the suhject in question will have no practical interest for tele- 

 scopes, unless the manufacturer of optical glass makes considerable 

 progress towards a better conformity in the dispersive powers of 

 crown and flint. 



In microscopic object-glasses the secondary dispersion, though 

 also perceptible, and the removal of which would be a consider- 

 able improvement, is not nearly so injurious as residual spherical 

 aberration, because the latter increases so rapidly with increase of 

 aperture. There can be no doubt therefore that objectives, even of 

 moderate aperture, would, by the correction of the chromatic dif- 

 ference of spherical aberration, attain a much higher standard of 

 defining power than has been hitherto obtained ; and if there be 

 microscopists who appreciate a luxurious exhibition of optical art, 

 they will, I hope, be gratified by seeing opticians apply this plan of 

 correction to objectives of various powers. But the practical advan- 

 tages of more complicated and more expensive constructions will 

 remain somewhat questionable in the case of those moderate aper- 

 tures, the full performance of which can be obtained (without 

 inconvenience from too short a focal length), with good ordinary 

 objectives. For a moderate aperture needs only a moderate ampli- 

 fication for the full exhibition of the minutest details accessible 

 to the aperture. Such an amplification may be obtained by an 

 objective of relatively long focal length, if the dioptric refinement 

 admits of a large increase of amplification by means of deep 

 eye-pieces ; in the other case it must be got by an objective of 

 shorter focal length, requiring low eye-pieces only for the same 

 amplification. Though from the purely optical point of view the 

 former is the more perfect instrument, the latter will be prac- 

 tically the same, as long as a moderately short focal length is 

 still sufficient for the aperture. Now even the largest aperture 

 attainable by dry lenses will need no higher amplification for perfect 

 exhaustion of the microscopic image than may readily be obtained by 

 a good g- of the ordinary construction combined with rather low eye- 

 pieces ; at all events no observation of any scientific value has ever 

 been made by any dry lens, which could not have been made just as 

 well with an §- ; and there is no practical inconvenience in the 

 case of such a glass, if it be properly made, which would not occur 

 to the same extent with a more accomplished |, fit for replacing 

 it by means of deeper eye-pieces. The more substantial advantages 

 will, therefore, be derived from the removal of residuary aberrations 

 for the higher apertures alone, which are obtained by the immersion 

 system, and especially in the case of homogeneous immersion ; for 

 in these cases higher amplification is needed than can be efiectively 

 obtained by an ^ with the ordinary mode of correction. Kaising 

 the level of dioptric performance in those objectives will remove 



