THE COMPOUND MICROSCOPE 



109 



mental principle of the compound microscope. The first lens, or objective, forms 

 an inverted image 8 times the size of the object. 



IH_F'I F'l /' 2 



(For 



i 

 - or 



= -8. 



o BA~BA~F'B~F'I-BI~/'-P~2-2i 



The negative sign shows that the image is inverted.) 



The second lens or eyepiece is now placed 2 in. from this image, i.e. 20 in. from 



FIG. 101. 



the objective, so that the inverted image ba is in its first focal plane. Consequently 

 the image ba will be seen under a magnification of ^ =-^ or 5. The total magnifica- 

 tion will be therefore - 8 x -^ = - 40. 



It follows, then, that by this arrangement the working distance is increased from 

 1 in. to 2 in., that the magnification is increased from 10 to -40, while the 

 errors from spherical and chromatic aberration are rather less than with a single 

 biconvex lens of 1 in. focus. Indeed, with the compound instrument, what is 

 called " pincushion distortion " would be almost entirely obviated. If an object 

 were a network of squares, it would be found that, on using a simple magnifying 

 lens, it would present the appearance shown in fig. 102 (pincushion distortion), 

 owing to the fact that the peripheral parts of the object would be more magnified 

 than the central parts. When however a compound instrument is used such as 

 that just described, the objective forms a real image showing " barrel-shaped 

 distortion " as in fig. 103, because the peripheral parts are less magnified than 



FIG. 102 

 Pincushion distortion. 



FIG. 103 

 Barrel-shaped distortion. 



the central parts. On now viewing this through the second lens, or eyepiece, the 

 barrel- shaped distortion will be completely corrected by the tendency of the virtual 

 image to suffer from pincushion distortion, so that the final image will be 

 rectangular. The fact that it is inverted is no inconvenience. 



The enormous advantage of a compound instrument is sufficiently obvious from 

 this simple illustration, but it must be remembered that the eyepiece only magnifies 



