File:Report and Transactions of the Glasgow Society of Field Naturalists (1873) (14779711504).jpg

Identifier: reporttransactio73glas (find matches)
Title: Report and Transactions of the Glasgow Society of Field Naturalists
Year: 1879 (1870s)
Authors: Glasgow Society of Field Naturalists
Subjects:
Publisher: Glasgow: Glasgow Society of Field Naturalists
Contributing Library: Natural History Museum Library, London
Digitizing Sponsor: Natural History Museum Library, London

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e focus. Largeangled object-glasses are dearer than low angled ones. The formershow surface markings best, owing to the greater number ofoblique rays which pass through them, whilst glasses with smallerangular aperture show various planes of the object to be at thesame time in focus. Object-glasses of moderate angular apertureare preferable, unless the microscopist be engaged in special studiesrequiring a large angled glass, but such may be provided withstops to reduce the angle in order to increase the penetration.Wide angled glasses give more light than narrow angled ones,but the latter have a greater working distance. Low powers are all glasses from the 5-in to the f-in.; mediumpowers from the ;-in., to the -;-in.; and all object-glasses of shorterfocus are called high powers. The simplest object-glasses are single bi-convex (PI. IIL fig. 1) orplano-convex lenses; if achromatic, they consist of a bi-convex and aplano-concave lens (fig. 3). In low compound objectives there are s^/.m
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^.1^. GLASGOW SOCIETY OF FIELD NATURALISTS. 125 two systems of lenses, the anterior, consisting of a plano-convexfront lens and a concavo-convex lens cemented together; theposterior combination consists of a plano-concave and bi-convex,(fig. 4) or of a bi-concave lens with a bi-convex lens above andbelow (fig. 5). In higher powers these lenses become morenumerous, consisting of a triple anterior and posterior and adouble middle combination as shown in diagram No. 8. Mr. F.H. Wenham has proved that a pencil of rays exceeding an angleof 40° from a luminous point, cannot be secured with less thanthree superposed lenses of increasing focus and diameter, but bythe use of such a triple combination rays beyond this angle canbe transmitted with successive reflections in their course towardsthe posterior conjugate focus. The first object-glass made byMr. Ross in 1831, consisted of three bi-convex lenses of crownglass, each of which was achromatized by its own plano-concavelens of flint glass

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