816 Transactions of the Society. 



and artificial chemical preparations, selecting those fluids similar to 

 crown, or light flint, in respect to refraction and equal to heavy flint 

 in respect to dispersive power. Substituting a collective (convex) lens 

 of such a fluid for one of ordinary crown (not a dispersive one for 

 flint, as has been done for quite another purpose) and combining it 

 with a concave lens of suitable flint of greater refraction and less 

 dispersion, the dispersive power of the flint may be balanced by 

 the dispersion of the fluid and from the difi*erence of the refractive 

 indices a sufficient amount of negative spherical aberration may 

 be obtained, the chromatic variation of which is just opposite to 

 that of ordinary binary lenses. At the same time the rapid 

 increase of dispersion from red to blue in some of these fluids, if 

 properly selected, will afibrd a simple and efiective expedient for an 

 almost perfect compensation of the secondary chromatic defects 

 arising from the ordinary flint and crown lenses. The fluid 

 uniting with the crown will make up the deficiency in the relative 

 dispersion of the blue part of the spectrum of the crown in com- 

 parison with flint. Owing to this peculiar fact the plan indicated 

 a degree of refinement of both corrections which, without taking 

 into consideration the practical difficulties, could not be obtained, 

 by ordinary means, at the time.* 



In 1873 Mr. Zeiss made an objective on the plan in question ; 

 a dry ^ (6 mm. focal-length) with a numerical aperture of 

 0'83, and another one in 1876, an immersion ^ (S'O mm.) of 

 1 • 15 aperture, a mixture of oil of cassia and of aniseed being used 

 in the former, and pure Cinnamyle-hydrogen (C^**H*0") in the 

 latter. The refractive indices and the partial dispersions of all 

 the materials having been measured very accurately, the formulae 

 were computed on the condition of perfect collection of spherical 

 aberration for the two colours, D and F, and of very approxi- 

 mate correction of the axial rays for the three colours, B, E, 

 and G. Both objectives were planned as quadruple systems, the 

 latter with duplex front. The three anterior lenses are not 

 essentially difierent from the ordinary construction ; the back lens 

 was made in each case of two glass lenses, flint and crown, including 

 between them a fluid meniscus of collective character. Both 

 objectives — which are still preserved at Mr. Zeiss' workshop — 

 perform exceedingly well when their fluid contents are kept in good 

 order. They yield images almost perfectly colourless on every kind 

 of preparation and with every kind of illumination, with very 

 superior definition — demonstrating the great improvement in 



* Objectives have several times been announced as being free from secondary 

 colours, owing to the application of three different kinds of glass. Anybody who 

 has a clear notion of the conditions of achromatism will see at once, that such an 

 assertion must have been either illusion or deception. For no Idnd of glass 

 hitherto produced would admit of the collection of three different colours in one 

 focus, even in a small telescopic objective. 



