. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . ^Fxo. 14.—Herschelian reflector. Herschel to avoid the loss of light from reflexion in the small mirrorof the Newtonian telescope. It has several disadvantages. (l)Theupper part of the observers head must necessarily obstruct seme ofthe rays which would otherwise fall on the large mirror; but whena telescope of very large aperture is employed the loss of light thusoccasioned is comparatively insignificant. Moreover, disturbanceof the air in front of the telescope is created by heat from theobservers hea
. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . ^Fxo. 14.—Herschelian reflector. Herschel to avoid the loss of light from reflexion in the small mirrorof the Newtonian telescope. It has several disadvantages. (l)Theupper part of the observers head must necessarily obstruct seme ofthe rays which would otherwise fall on the large mirror; but whena telescope of very large aperture is employed the loss of light thusoccasioned is comparatively insignificant. Moreover, disturbanceof the air in front of the telescope is created by heat from theobservers head and body, and this is fatal to the best avoid the latter drawback Sir John Herschel (Ency. Brit., 8thed., art. Telescope, vol. xxi p. 128) suggested the employment ofa smaJl right-angled prism of total reflexion placed close to the eye-lens of the eye-piece, to permit the observer to view the image bylooking in a direction at right angles to the eye-piece, and thereforeat right angles to the tube. (2) In consequence of the tilting of themirror aberrarion is created, an
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