68 



THEORY OF THE MICROSCOPE. 



Hence, probably, the opticians now construct their medium 

 objectives for cover-glasses of about J mrn. in thickness, whilst the 

 objectives of Amici were expressly intended for use with cover- 

 glasses of various, and in some cases very considerable, thickness 

 (up to 1'5 mm.). 



VII. 



THE FLATNESS OF THE FIELD OF VIEW. 



ON observing a straight line through a lens, or system of lenses, 

 its image appears to be straight only when it passes through the 

 centre of the field of view ; in every other position it will appear 

 curved, with its convex side towards the centre, and the curvature 

 varies with the distance from the centre. A mesh-work of squares 

 (Fig. 28) will accordingly appear as represented in Fig. 29, and 



FIG. 29. i 



FIG. 28. 



every other object in the virtual image will appear more or less 

 distorted in a similar manner. 

 a' 



y"" FIG. 30. 



It was formerly customary to explain these distortions as due to 

 the fact that the points of the virtual image a I' (Fig. 30), which 



