32 PRACTICAL TREATISE ON 



the second bending, in the opposite direction, by the third 

 lens." This combination, though called by Mr. Holland a 

 triplet, is essentially a doublet, in which the anterior lens is 

 divided into two, and is capable of transmitting a pencil of 

 65°. Here we must take our leave of the history of the single 

 microscope, and commence that of the achromatic compound 

 instrument. 



Notwithstanding the great improvements which had taken 

 place in the compound microscope during a period of nearly 

 two centuries, we find, says Mr. Eoss,* that it was " a com- 

 paratively feeble and inefficient instrument, owing to the 

 increase in the chromatic and spherical aberrations occasioned 

 by the great distance through which the light had to pass. 

 The image formed by the object-glass was not a simple one, 

 but made up of an infinite number of variously coloured and 

 variously sized images. Those nearest the object-glass would 

 be blue, and those nearest the eye-glass would be red. The 

 effect of this being the production of so much confusion, that 

 the instrument was reduced to a mere toy, although these 

 errors were diminished to the utmost possible extent by 

 limiting the aperture of the object-glass, and thus restricting 

 the angle of the pencil of light from each point of the object. 

 But this proceeding made the picture so obscure, that, on the 

 whole, the best compound instnunents were inferior to the 

 simple microscopes having a single lens, with which, indeed, 

 almost aU the more important observations of the preceding 

 century were made." The compound microscope, in its 

 chromatic condition, having been found to be incapable of 

 further advancing in a right way scientific research, many 

 artists of eminence applied themselves to the work of im- 

 provement; we are told that achromatism had been discovered 

 in 1729 by a private gentleman in Essex, named Chester 

 More Hall, who, in 1733, constructed and applied to a teles- 

 cope an achromatic object-glass, having been led to its dis- 

 covery by the study of the human eye, and by finding that 

 two kinds of glasses combined, refracted light without decom- 

 posing it. Two of his achromatic telescopes were for a long 

 * Op. Cit., p. 6. 



