A. E. CONHADY 63 



by microscopists for all work for which they are sufficiently powerful. 

 In the ordinary ternary objectives of the 1/6 inch type, with approx- 

 imately plano-convex components, the curvature of tlie field is also 

 of reasonably moderate amount. But it is a general experience 

 that highly corrected objectives are very much worse as regards 

 curvature of field. In the light of my most recent work on the 

 general theory of lenses (Monthly Notices, November, 1919), this 

 curious and objectionable peculiarity is easily explained, and becomes 

 revealed as a 7iecessary consequence of high spherical and chromatic 

 correction if the usual number of components is adhered to. In 

 the Lister and Amici types of ordinary objectives, which are fairly 

 satisfactory as regards curvature of the field, the front lens is of 

 such a form as to produce strong outward coma and there is in the 

 back lens or lenses a corresponding amount of inward coma. The 

 simple extensions of Seidel's theory given in the paper last referred 

 to show that this is the state of affairs which tends to diminish 

 undercorrected astigmatism or even to reverse it into the more 

 desirable over-corrected form. High correction of the zonal spherical 

 aberration, and to a still greater extent complete removal of the 

 spherical variation of chromatic correction necessitate a more or 

 less complete reversal of the coma effects in front and back com- 

 ponents. In other words, with the usual types of objectives, reduction 

 of curvature and apochromatic or semiapochromatic correction are 

 completely antagonistic and incompatible : what benefits one correc- 

 tion is detrimental to the other. Fortunately the extended theory 

 also indicates a way out of this dilemma. It appears fairly certain 

 that by building the objective itself on the lines required by the 

 apochromatic condition, but leaving it spherically undercorrected, 

 perhaps also chromatically overcorrected to a moderate extent, and 

 with a considerable amount of outward coma (this is the most 

 important), and by correcting these residuals in a widely sejparated 

 additional back lens, it will be possible to combine moderate curvature 

 of field with apochromatic perfection and thus to remove the worst 

 outstanding defect of the best objectives. 



Condensers for the proper well-regulated illumination of micro- 

 scopic objects are identical in optical design with objectives, the only 

 difference being that the light passes through in the reverse direction 

 and that a lower degree of correction is sufficient not only on 

 theoretical but also on practical grounds, for nearly always condensers 

 are used in conjunction with the " plane " mirror, which invariably 

 is very far from optical perfection, and so introduces irregular aberra- 

 tions of unknown magnitude and kind, and moreover the light from 

 the condenser has to pass through the slide on which the object is 

 placed. This slide is practically little better than window glass as 

 far as optical quality and perfection of surfaces is concerned, and 

 the great variation in thickness is another source of imperfection, 

 especially with dry condensers of high N.A. 



Moderate amounts of residual aberrations in condensers can 

 always be effectively neutralised by using a sufficiently large source 

 of light of uniform brightness or by magnifying the source by a 

 sufficiently well-corrected " bull's-eye," if the source of light is 

 naturally small. 



