GENEKAL PKELIMINAKY PKEPAEATIONS 195 



Apochromatic objectives are in general much more accu- 

 rately described than achromatic ones, so that the margin of 

 error in estimating magnification solely in reference to the 

 indicated focal length of the objective is likely to be quite 

 small. Their focal length is described in millimetres, and the 

 chief ones are indicated in the foregoing table. The magnifica- 

 tions set out are their initial ones, and on placing an eye-piece 

 on the microscope an image projected at ten inches' distance 

 from it will be as many times larger as the initial power of the 

 eye-piece ; that is, assuming that we take a No. 2 projection- 

 ocular, the magnification at ten inches' distance from the front 

 lens of the projection-ocular will be twice that to be obtained 

 without the latter. When the image is projected, e.g. with 

 apochromatic objectives and no ocular, the initial magnifica- 

 tion indicated is obtained at a distance of 160 mm., or in the 

 case of long-tube objectives 250 mm. from the objective end 

 of the microscope body-tube. When a projection-ocular is 

 added, the image, magnified 2, 4, or more times according to 

 the ocular, is then obtained at a distance of 160 mm., or 250 

 mm. measured from the ocular itself and not from the objective. 



With projection-oculars it is fairly safe to assume that 

 their magnification is in accordance with the number engraved 

 on them, so that a x 2 or X 4 project ion- ocular will magnify 

 the image two or four times as indicated. But should the 

 ordinary eye-piece be used, indicated by the letters A, B, C, and so 

 on, or by some arbitrary number, then the only way is to determine 

 their magnification by the projection of a micrometer ruling. 



The influence of camera extension is very similar in effect 

 to a change of ocular. If the ground-glass screen is ten inches 

 from the front lens of the ocular, we then obtain magnification 

 in proportion to the focal length of the objective, multiplied 

 by the number of times that the ocular magnifies. When the 

 camera is extended beyond the ten inches, the amplification 

 is then directly proportional to the increase of camera exten- 

 sion. For instance, if we place an object on the stage with a 

 one-inch objective and, say, a No. 8 eye-piece, we should find 

 the object to be magnified approximately 80 times at ten inches' 

 distance. But if we double the camera-length we should now 

 get an object twice that size, equal to a magnification of 160 times. 



It is most convenient on any photo-micrographic apparatus 



o 2 



