Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/280

264 MICROSCOPE important discovery was made by Mr Andrew Boss that a very decided difference exists in the precision of the image according as the object is viewed with or without a covering of thin glass, as also according as this cover is thin or thick. 1 As this difference increases in proportion to the widening of the aperture, it would obviously be a Fig. 3. i flilci t * . <i t i * , -- i * * * t t t Fig. 9. Fig. 10. Fig. 11. Portions of Siliceous Valve of Pleurosigma angulatum, from a Photo graph taken by Central Illumination. Magnified 2000 diameters. source of great error and embarrassment if a means could not be found for its rectification. Its optical source, how ever, having been found by Mr Ross to lie in the " negative aberration " which is produced in the rays proceeding from the object to the front glass of the objec tive by the interposition of the plane-glass cover, and which increases with its thickness, his practical ability enabled him at the same time to indicate the remedy, which consists in under-correcting the front lens and over- correcting the two pos terior combinations, and in making the distance between the former and the latter capable of adjustment by means of a screw-collar, as shown in fig. 12. For when the front pair is approxi- B mated most nearly to the next, and its distance from the object is in creased, its excess of positive aberration is more strongly exerted upon the other two pairs than it is in the con- FIG. 12. Section of Adjusting Achromatic trary conditions, a nd b J ect - Glass> A, uncovered ; B, covered, thus neutralizes the negative aberration produced by the interposition of the covering-glass. This correction is not needed for objectives of low or medium power and small angle of aperture ; but it should always be provided when the angle exceeds 50, unless (as is now generally done 1 Trans. Soc. of Arts, vol. li. in the case of objectives constructed for students use) the maker adjusts them originally, not for uncovered objects, but for objects covered with glass of a standard thickness, say O005 or O CKM inch. A departure from that standard to the extent of one or two thousandths of an inch in either direction, though extremely injurious to the performance of objectives whose aperture is 125 or more, scarcely makes itself perceptible in those of 90 or 100. And the same may be said in regard to the immersion- objectives next to be described, which are peculiarly suitable to the purposes of minute histological research. Immersion System. It was long since pointed out by Professor Amici that the introduction of a drop of water between the front surface of the objective and either the object itself or its covering-glass would diminish the loss of light resulting from the passage of the rays from the object or its covering-glass into air, and from air into the front glass of the objective. It was obvious to him, more over, that when the rays enter the object-glass from water, instead of from air, both its refractive and its dispersive action will be so greatly changed as to need an important constructive modification to meet the new condition. This modification seems never to have been successfully effected by Amici himself ; but his idea was taken up by the two eminent Paris opticians, MM. Hartnack and Nachet, who showed that the application of what is now known as the "immersion system " to objectives of short focus and large angular aperture is attended, not merely with the advan tages expected by Professor Amici, but with others on which he did not reckon. As the loss of light by the reflexion of a portion of the incident rays increases with the obliquity of their incidence, and as the proportional loss is far smaller when the oblique rays pass into glass from water than when they enter it from air, the advantage of increas ing the angular aperture is more fully experienced with "immersion" than with "dry" objectives, just as Professor Amici anticipated. But, further, the immer sion system allows of a greater working distance between the objective and the object than can be attained with a dry or air objective having the same angular aperture ; and this increase affords not only a greater freedom of manipulation, but also a greater range of " penetration " or " focal depth." Further, the observer is rendered so much less dependent upon the exactness of his cover- correction that it is found that water-immersion objectives of high power and considerable angular aperture, extremely well adapted for the ordinary purposes of scientific inves tigation, can be constructed without it, a small departure from the standard thickness of covering-glass to which such objectives are adjusted by the maker having scarcely any effect upon the distinctness of the image. It is now the practice of several makers to supply two fronts to objectives of y^j-or T -inch focus, one of them fitting the objective for use " dry " (that is, in air), whilst the substitution of the other converts it into a water-immersion objective. And in the objectives constructed on Mr Wenham s system no change in the front glass is needed, all that is necessary for making them work as immersion-lenses being a yet closer approximation of the front lens to the second combination, which can be made by the screw-collar. Within the last few years, however, the immersion system has undergone a still further and most important development, by the adoption of a method originally suggested by Mr Wenham (though never carried out by him), and independently suggested by Mr Stephenson to Professor Abbe of Jena, under whose direction it was first worked out by Zeiss (the very able optician of Jena), who has been followed by Powell and Lealand of London, as

well as by several other constructors of achromatic objec-