. La cyclopaedia d'anatomie et physiologie. Anatomie ; physiologie ; Zoologie. 338 MICROSCOPE. un pied d'agrandissement, et l'ouverture pourrait admettre de grande extension sans pro- proportionnelle à la forme sphérique et chro- matique des aberrations. Cette suggestion a été adoptée en pratique avec un succès complet en ce qui concerne la performance de verres exécutés sur ce plan ; mais les difficultés de toutes sortes dans la voie de leur exécution sont de nature à les rendre très cher ; et comme ils ne sont pas supérieures à la combinaison maintenant pour être décrites, ils ont récemment été très super- seded b

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. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 338 MICROSCOPE. an equal magnifying power, and the aperture would admit of great extension without a pro- portional increase in the spherical and chro- matic aberrations. This suggestion has been carried into practice with complete success as regards the performance of lenses executed on this plan; but the difficulties of various kinds in the way of their execution are such as to render them very expensive; and as they are not superior to the combination now to be de- scribed, they have latterly been quite super- seded by it. This combination was first proposed by Dr, Wollaston, and is known as his doublet. It consists of two plano-convex lenses, whose focal lengths are in the proportion of one to three, or nearly so, having their convex sides directed towards the eye, and the lens of shortest focal length nearest the object. In Dr. Wollaston's original combination no stop was interposed, and the distance between the lenses was left to be determined by experiment in each case. A great improvement was subse- quently made, however, by the introduction of a stop between the lenses, and by the divi- sion of the power of the smaller lens between two; this is due to Mr. Holland.* By these means a combination may be produced, in which the errors are made to correct each other so nearly that all the advantages of a wide aperture with a very short focus may be gained. The general nature of the performance of a doublet or triplet may be understood from the adjoining figure, (figAGO, } in which L 0 L' is Fig. 160.. u Diagram of the passage of rays through a doublet, the object, P a portion of the pupil, and D D the stop. The pencils of light from the two extremities, L L', of the object cross each other in the stop, and consequently pass through the two lenses on the opposite sides ' Trans, of Soc. of Aits, vol. xlix. of the axis O P ; so that each becomes af- fected by opposite errors, which to a certai

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