. A practical treatise on the use of the microscope, including the different methods of preparing and examining animal, vegetable, and mineral structures. Microscopes; Microscopy. MAGNIFYING POWERS. 161 fering with definition. The spherical aberration may be con- siderably diminished by attending to the figm^e of the lens employed; thus, if it be a planoconvex, the convex side should be placed towards the eye, if a double convex, it has been found in practice that one whose radii are in the proportion of one to six is the form in which the aberration is the least; but it can be entirely got ri
. A practical treatise on the use of the microscope, including the different methods of preparing and examining animal, vegetable, and mineral structures. Microscopes; Microscopy. MAGNIFYING POWERS. 161 fering with definition. The spherical aberration may be con- siderably diminished by attending to the figm^e of the lens employed; thus, if it be a planoconvex, the convex side should be placed towards the eye, if a double convex, it has been found in practice that one whose radii are in the proportion of one to six is the form in which the aberration is the least; but it can be entirely got rid of by combinations of lenses so dis- posed, that their opposite aberrations may correct each other; this was first accomplished in a satisfactory manner by the doublet of Dr. Wollaston, before described in pages 30 and 65, as consisting of two planoconvex lenses whose focal lengths are in the proportion of one to three, the lens of shortest focus being placed next the object, and the convex surfaces of both directed towards the eye, with a stop or diaphragm between them. "Wollaston did not employ the stop, as his doublets were of such high power, that the lenses nearly touched each other. The action of the doublet wiU be best understood by the diagram, fig. 114, copied from Mr. Eoss's article " Microscope," 1 before alluded to, where P represents a portion of the pupil, D D the dia- phragm or stop, and L O L' the object. Each of the pencils of light from the extremities of the object L L' is rendered eccentric by the stop, and the ray that passed through the first lens near to the cen- tre is made to pass through the periphery of the se- cond lens, and on the op- posite side of the common axis, P O; thus each is affected by opposite errors which, in some measure, "neutralise each other, and the rays, R B, R B, emanating from L, being bent to the 11. Please note that these images are extracted from scanned page images that may have been digitally enhanced f
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