162 THE MICROSCOPE AND ITS KEVELATIONS. 



15), especially the former; and it is an attribute essential to the satisfac- 

 tory performance of any Objective, whatever be its other qualities. 

 Good definition may be more easily obtained with lenses of small or 

 moderate, than with lenses of large angular aperture; and as it is impos- 

 sible to construct ' dry ' Objectives of very wide angle, without some 

 sacrifice of perfect correction (Abbe), there is a limit which, where 

 * definition' is of primary importance, cannot be advantageously passed. 

 On the immersion system, however, and especially on the ' homogeneous 

 immersion' system ( 19, 20), Objectives can be constructed of very 

 much wider angle, without any injurious sacrifice of definition arising 

 from inadequate correction. But here there comes in another source of 

 impairment the difference in the perspective views of every object not a mere 

 mathematical point or line, which are received through the different parts 

 of the area of the Objective. The picture given by the entire area is so 

 to speak the 'general resultant' of the dissimilar pictures recived 

 through these several parts; 1 and as this dissimilarity obviously increases 

 with the angle of operture of the Objective, its defining power must be 

 proportionately impaired. This theoretical conclusion has been experi- 

 mentally verified by Dr. Eoyston Pigott; who has found that by com- 

 paring Objectives of large with those of moderate apertures, on such 

 objects as the cracks in Mr. Slack's silica-films ( 152), or the aerial image 

 formed by the Achromatic Condenser of a hair stretched before the light 

 at some distance, the advantage was decidedly on the side of the latter. 

 He has shown 2 "that the black margins or black marginal annuli of 

 refracting * spherules constantly displayed by small aperture Objectives, 

 are attenuated gradually to invisibility as the apertures are widened to the 

 utmost; that the black margins of cylinders, tubules, or semi-tubules, 

 also suffer similar obliterations; and that, in consequence, minute details 

 are concealed or destroyed till the aperture is sufficiently reduced." It is 

 also the experience of Messrs. Dallinger and Drysdale, that for the 

 definition of the immeasurably-minute reproductive granules of the 

 Monadine forms whose life-history they have studied ( 418), or of the 

 flagella of Bacterium termo ( 305), which may be characterized as the 

 highest feats of Biological Microscopy yet performed, moderate angles of 

 aperture are unquestionably *o be preferred (vi.) An experienced Micro- 

 scopist will judge of the defining power of an Objective by the quality of 

 the image it gives of any fitting object with which he is familiar; no test 

 being, in the Author's judgment, more suitable than the Podura-scale 

 ( 162). Any imperfection in Defining power is exaggerated, as already 

 pointed out ( 26, 136), by 'deep Eye-piecing;' so that, in determin- 

 ing the value of an Objective, it is by no means sufficient to estimate its 

 performance under a low Eye-piece an image which appears tolerably 



1 This point has been long kept before the mind of the Author, by his studies 

 in Stereoscopic Microscopy ; the condition of the effect of relief in the Binocular 

 image being the dissimilarity of the pictures of any object not absolutely flat, 

 that are formed by the right and the left halves of the Objective respectively 

 ( 39). And he is glad to find his view of its importance confirmed by so able a 

 practical Optician as Mr. Zentmayer ; who, in a Lecture on the Elementary Prop- 

 erties of Lenses, published in the " Journal of the Franklin Institute " for May and 

 June, 1876, and cited in the " Monthly Microscop. Journ.," Vol. xvi. (1876), p. 317, 

 called attention prominently to the confusion of images necessarily attendant 

 upon large apertures, except when viewing absolutely flat objects, from the fact 

 that the image formed by pencils transmitted by one side of the lens are unavoid- 

 ably different from corresponding images formed by the opposite side of the lens. 



2 " Proceeding of Royal Society," June 19th, 1879. 



