The London, Edinburgh and Dublin philosophical magazine and journal of science . rre-sponding to its grasp, when the condenser was itself adjustedat such a distance from the microscopic object as to produceone uniform colour throughout that disk of light. Upon theslit the image of an incandescent gas-burner had beenthrown, which was thus the source of light. The mirror ofthe microscope was then turned until the colour seen inimage x was bluish green. With these arrangements weknow that the wave-lengths we are using are in the neigh-bourhood of X=0-52yu-. We are now in a position to determine w
The London, Edinburgh and Dublin philosophical magazine and journal of science . rre-sponding to its grasp, when the condenser was itself adjustedat such a distance from the microscopic object as to produceone uniform colour throughout that disk of light. Upon theslit the image of an incandescent gas-burner had beenthrown, which was thus the source of light. The mirror ofthe microscope was then turned until the colour seen inimage x was bluish green. With these arrangements weknow that the wave-lengths we are using are in the neigh-bourhood of X=0-52yu-. We are now in a position to determine what size of stopit will be advisable to introduce under the condenser, so as toblock out most of those direct beams whose diffracted lightdoes not come within the grasp of the objective, and whichaccordingly would be not only useless but prejudicial. Since\=052/a and o-=0265 fi we find by equation (2) that<? + </ = 1*96. Hence when g has its largest value, which isT25, g will have its smallest, which accordingly is </=Q-71, Dr. G. J. Stoney on Microscopic Vision. 509. We should therefore put in a stopwhich will block out all beams thathave a grasp less than this *. Theimage at x will then have the ap-pearance represented on a large scalein the annexed diagram. An annulusof bright light will be seen in itbetween the edge of the stop, whichlies at ^/ = 071, and the limit ofthe grasp of the condenser at G^O85,and faint light will be seen farther out to the limit of thegrasp of the objective at G = l*25. We may, however, with advantage block out still more ofthe light. For the specks upon the diatom are numerous,and although not quite regularly disposed, they are approxi-mately in straight vertical rows, and less regularly in some-what wavy horizontal rows. In this description we suppose Image x, with X scale. A 0 o Z>o z o Oct 0 d o
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