1878.] determining the limits of microscopic vision. 219 



the black diffraction ring measured nearly tlie same as the wave 

 length in Fraunhofer's line F, or nearly one-fourth of the diameter 

 of the disk, go^ooth. But since the aerial solar image, 1000 inches 

 distant, was on the whole diminished 1000 times, the correct 

 diameter of the stage-disk should have been 3 5 q q o th of an inch^ 

 Whereas the spurious disk measured about twice this amount. This 

 result shews that the Helmholtz law for diffraction was obtained, 

 or rather was sustained, notwithstanding the great number of 

 lenses employed, nearly twenty between the sun and its final 

 image. Moreover the law ceased to operate when the objects were 

 only mildly illuminated. It is reasonable therefore to conclude 

 the miniature image of the spider-lines is sufficiently exempt from 

 spurious enlargement due to brilliant diffraction disks or brilliant 

 lines. Other combinations also confirm this point. Spherical 

 aberration in the glasses causes the lines to be blurred and thick- 

 ened or rendered invisible. But when an objective possesses the 

 high excellence of presenting an image of a fine fibre sharply, 

 I may say exquisitely defined, when magnified one thousand times, 

 it is plain by reversing the rays a miniature may be formed with 

 great precision which will bear great magnification. 



As the experiment requires some practice, it may be well for 

 the observer to commence with easier miniatures, such as hairs 

 or spun glass placed four or five inches below the stage, and if 

 possible illuminated by a plane mirror reflecting daylight, such as 

 a white cloud or grey sky usually affords. An opal globe shading 

 the glare of a parafin flame is also a . good back-ground. By 

 degrees, as experience is gained, he may at last ascend to the 

 higher definition, such as that excellently obtained by Messrs 

 Beck's -^Q immersion, giving a -miniature at seven- inches interval 

 diminished 140 times. Figs. 2, 3, 4. 



We will now suppose the experiment is ready to be made by 

 the complete adjustment of the coincidence of the axes of the 

 two objectives, the one above and the other below the stage, either 

 " immersion or dry lenses." The preliminary must now be per- 

 formed of finding the absolute diminution of the miniature for 

 different sets of glasses. And here I should premise that the 

 observing glass in general should not exceed in power one half 

 that of the miniature glass, that is to say, "one quarter" should 

 be used to view the miniature formed by an eighth, unless the 

 latter is of most exquisite excellence, otherwise the spherical errors 

 of the latter will be made horridly visible by the excellence of the 

 observing microscope. 



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