The microscope and its revelations . h meet at the widest angle.—Inusing either of these illuminators, the rays which are madeto fall upon them should be parallel, consequently the planemirror should always be employed; and when, iastead ofthe parallel rays of daylight, we are obliged to use thediverging rays of a lamp, these should be rendered as parallelas possible, previously to then: reflection from the mirror,by the interposition of the bulls-eye condenser (§ 64) soadjusted as to produce this effect. 62. Eor the exhibition of those classes of objects which aresuitable for black-^rouud lUu
The microscope and its revelations . h meet at the widest angle.—Inusing either of these illuminators, the rays which are madeto fall upon them should be parallel, consequently the planemirror should always be employed; and when, iastead ofthe parallel rays of daylight, we are obliged to use thediverging rays of a lamp, these should be rendered as parallelas possible, previously to then: reflection from the mirror,by the interposition of the bulls-eye condenser (§ 64) soadjusted as to produce this effect. 62. Eor the exhibition of those classes of objects which aresuitable for black-^rouud lUumuiation, and which are betterseen by Hght sent into them from every azimuth, than theyare by a pencU, however bright, incident in one directiononly, no more simple, convenient, and efficient means couldprobably be found, than that which is afforded by the spottedlens for low powers, and by the parabohc illuminatorfor powers as high as l-4th or l-5th of an inch focus;—theuse of the latter with the highest powers, being rendered. Parabolic Illmmnator. PAEAIBOLIC ILLIJIillNATOE.—POLAIlIZDir& APPAEAirS. 135 disadvantageous by the great reduction in the amount of light,occasioned by the necessity for cutting-off of all the raysreflected from the paraboloid, which fall upon the objectTritliin the limits of then: angle of aperture.—One of thegreat advantages of this kind of iLlumiiiation consists inthis ; that, as the light radiates from each part of the objectas its proper source, iastead of merely ^(^.y^?/^^ through it froma more remote source, its different parts are seen much morein their normal relations to one another, and it acquires farmore of the aspect of solidity. The rationale of this is easilymade apparent, by holdiag-up a glass vessel with a figuredsurface between one eye and a lamp or a wiadow, so that itis seen by transmittecl light alone; for the figures of itstwo surfaces are then so blended-together to the eye, thatunless theu- form and distribution be pre
Size: 1149px × 2174px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, booksubjectmicroscopes, booksubjectmicroscopy
