THE MICBOSCOPE. 21 



rays L E are part of the cone of rays emanating from B, and 

 the angle L E M is that under which the eye will see the mag- 

 nified image of the arrow, which is evidently many times 

 greater than the arrow could be made to occupy in the naked 

 eye at any distance within the limits of distinct vision. The 

 magnifying power depends on two circumstances: first, on the 

 ratio between the anterior distance A or B D and the poste- 

 rior focal length C B' or D A'; and secondly, on the power of 

 the eye-lens L M. The first ratio is the same as that between 

 the object A B and the image A' B'; this and the focal length 

 or power of the eye lens are both easily obtained, and their 

 product is the power of the compound instrument. 



Since the power depends on the ratio between the anterior 

 and posterior foci of the object-glass, it is evident that by in- 

 creasing that ratio any power may be obtained, the same eye- 

 glass being used ; or having determined the first, any further 

 power may be obtained by increasing that of the eye-glass; and 

 thus, by a pre-arrangement of the relative proportions in which 

 the magnifying power shall be divided between the object-glass 

 and the eye-glass, almost any given distance (within certain 

 limits) between the first and its object may be secured. This 

 is one valuable peculiarity of the compound instrument; and 

 another is the large field, or large angle of view, which may be 

 obtained, every part of which will be nearly equally good; 

 whereas with the best simple microscopes the field is small, and 

 is good only in the centre. The field of the compound instru- 

 ment is further increased by using two glasses at the eye-end; 

 the first being called, from its purpose, the field-glass, and the 

 two constituting what is called the eye-piece. This will be 

 more particularly explained in the figure of the achromatic 

 compound microscope presently given. 



For upwards of a century the compound microscope, not- 

 withstanding the advantages above mentioned, was a compara- 

 tively feeble and inefficient instrument, owing to the distance 

 which the light had to traverse, and the consequent increase of 

 the chromatic and spherical aberrations. To explain this we 

 have drawn in Fig. 12 a second image near A' B', the fact being 

 that the object-glass would not form one image, as has been 

 supposed, but an infinite number of vaiiously-colored and vari- 



