. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. nation. BLACK FIELD ILLUMINATION log Black field illumination is obtained most perfedllyby means of a paraboloid placed beneath the Wenhams first contrivance consisted of a silveredparabolic cup, open at the apex, which reflecftedparallel rays very obliquely, the access of direcft lightbeing prevented by a stop placed in a proper performance of this illuminator was perfedl, theFig. 2g. / ^ c \\ r r r V r 7 only objedlion
. On microscopical manipulation : being the subject matter of a course of lectures delivered before the Quekett Microscopical Club, January-April, 1869. nation. BLACK FIELD ILLUMINATION log Black field illumination is obtained most perfedllyby means of a paraboloid placed beneath the Wenhams first contrivance consisted of a silveredparabolic cup, open at the apex, which reflecftedparallel rays very obliquely, the access of direcft lightbeing prevented by a stop placed in a proper performance of this illuminator was perfedl, theFig. 2g. / ^ c \\ r r r V r 7 only objedlion to it being that it was rather difficultto keep the reflecfling surface bright. This has beenremedied by constructing the paraboloid of glass, theilluminator now in use being a solid cast in glass ofthe original silvered speculum (Fig. 29). It refle(ftslight quite as well as if it were silvered, owing to the no MICROSCOPICAL MANIPULATION. law of internal reflection. When a ray of lightpassing through glass falls upon a surface the angleof which is not less than 38° 41, it cannot pass outof the glass into air, but is totally reflected (Fig. 30).. The ray, a, entering the triangular prism at h,passes through to c, where not being able to passout into air, it is refledled as if the surface at c weresilvered, only far more perfedtly, the refledlion beingtotal. This principle is made extensive use of in theconstrudlion of optical apparatus, a reflecting prismbeing always preferable when it can be used, onaccount of its wasting less light than a metallicspeculum or silvered glass mirror. If a tumbler isfilled with water, and the surface of the water lookedat upwards through the side of the glass, it will befound that the surface of the water will refle(5t lightmore brilliantly than the best mirror. The glass para-boloid, although unsilvered, and appearing at firstsight to be a lens rather than a mirror, is neverthelessa far more perfedl refledlor than its metallicpredecessor. Paral
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectmicroscopy, bookyear1
