The microscope; an introduction to microscopic methods and to histology . ocular andserving to give the image-forming rays from the objective an increased effective form of this accessory was made by Tolles, who made it as a smallachromatic concavo-convex lens to be screwed into the lower end of the draw-tube(frontispiece) and thus but a short distance above the objective. The divergencegiven to the rays increases the size of the real image about two-fold. no MAGNIFICATION AND MICROMETRY [CH. IV sfe H 4 \\ L^I— —h \ /1 • a A V °i ,\ Fig. ioo. Figure showingthe position of the mic
The microscope; an introduction to microscopic methods and to histology . ocular andserving to give the image-forming rays from the objective an increased effective form of this accessory was made by Tolles, who made it as a smallachromatic concavo-convex lens to be screwed into the lower end of the draw-tube(frontispiece) and thus but a short distance above the objective. The divergencegiven to the rays increases the size of the real image about two-fold. no MAGNIFICATION AND MICROMETRY [CH. IV sfe H 4 \\ L^I— —h \ /1 • a A V °i ,\ Fig. ioo. Figure showingthe position of the microscope,the camera lucida, the eye, andthe difference in size of the im-age depending upon the dis-tance at which it is projectedfrom the eye. (a) The size at25 cm.; (b) at35 cm., ($ 162). that the image could not be seen and measured at any other distance, butbecause some standard must be selected, and this is the most commonone. The necessity for the adoption of some common standard will beseen at a glance in Fig. 100, where is represented graphically the fact. Fig. 10i. Sectional viewof the Abbe Camera Lucidato show that in measuringthe standard distance of 250millimeters, one must meas-ure along the axis from thepoint P, at the left of theprism, to the mirror, andfrom the mirror to thedrawing surface. For afull explanation of thiscamera lucida, see nextchapter. Fig. ioi. that the size of the virtual image depends directly on the distance atwhich it is projected, and this size is directly proportional to the ver-tical distance from the apex of the triangle, of which it forms a distance of 250 millimeters has been chosen on the suppositionthat it is the distance of most distinct vision for the normal human eye. CH. IV] MAGNIFICATION AND MICROMETRY Demonstrate the difference in magnification due to the distance atwhich the image is projected, by raising the microscope so that thedistance will be 350 millimeters, then lowering to 150 millimeters. In preparing dr
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