312 W. .M. BALE OX THE 



they appioacli the edge of the field, and consequently being more 

 magnified. This will be best exemplified by making a diagram 

 similar to that given here (Fig. 1) but of the full size (which is 

 twice the diameter of the print). Assuming that the microscope 

 is placed in the usual position for drawing, A is the centre of the 

 eye-piece, and B C the surface for drawing on, the centre of which 

 must be 10 inches from A. The distance from B to C, 7 J inches, 

 represents the apparent diameter of the field, as projected on the 

 paper, this being the field of a Swift B ocular which gives the 

 largest field of any of my eye-pieces. Similarly D E, 41 inches, 

 represents the apparent field of a Swift A. We see at once that 

 rays apparently proceeding from about the margin have a con- 

 siderably longer path than those from the centre, and the exact 

 difference can be readily measured if we describe the arc F G. 

 having a radius of 10 inches from A. We see that while objects 

 at the centre of the field are drawn at a distance of 10 inches 

 those at B or G are drawn at about 10^ J ; their magnification 

 therefore exceeds that of objects at the centre in the same ratio 

 as that in which 10 exceeds lOyJ ; so that, supposing the central 

 part is magnified 100 diameters, the marginal part will be magni- 

 fied nearly 107, and the field-margin will appear farther from the 

 centre than it should. Let the arc F G he divided into 12 or 

 15 equal parts, and let lines be drawn from A through each of 

 the divisions to the line B C, and we shall see that the divisions 

 on B C are not all equal, those near the margins being slightly 

 longer, as the micrometer-divisions would be on a drawing. Now, 

 while the line B C is the apparent field-diameter as shown in a 

 camera-drawing, the real field-diameter, as taken at a uniform 

 distance from A of 10 inches, is the length of the curved line 

 F G. To measure this accurately the simplest way is to cut the 

 paper along F G and apply a flexible rule to the margin, when it 

 will be found that the field, which measures 1\ inches in the 

 camera-drawing, is really about 7fg-. Of course with an ocular of 

 small field the difference will be much less, as is conspicuously 

 shown in the diagram. 



Instead of making a diagram we may proceed as follows : 

 take a strip of cardboard and bend it to the curve of the arc F G, 

 place it under the camera and prop the ends, holding it in such 

 a position that every part will be 10 inches from the centre of 

 the eye-piece, then mark on it the positions of the opposite edge* 



