22 THE MIOBOSCOPE. 



ous-sized images, occupying the space between the two dotted 

 arrows. Those nearest the object-glass would be red, and those 

 nearest the eye-glass would be blue. The effect of this is to 

 produce 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 involved the defects, already 

 explained, of making the picture obscure, so that on the whole 

 the best compound instruments were inferior to the simple 

 microscopes of a single lens, with which, indeed, all the impor- 

 tant observations of the last century were made. 



Even after the improvement of the simple microscope by the 

 use of doublets and triplets, the long course of the rays, and 

 the large angular pencil required in the compound instrument, 

 deterred the most sanguine from anticipating the period when 

 they should be conducted through such a path free both from 

 spherical and chromatic errors. Within twenty years of the 

 present period, philosophers of no less eminence than M. Blot 

 and Dr. Wollaston predicted that the compound would never 

 rival the simple microscope, and that the idea of achromatizing 

 its object-glass was hopeless. Nor can these opinions be won- 

 dered at when we consider how many years the achromatic 

 telescope had existed without an attempt to apply its principles 

 to the compound microscope. When we consider the smallness 

 of the pencil required by the telescope, and the enormous in- 

 crease of difficulty attending every enlargement of the pencil 

 when we consider further that these difficulties had to be con- 

 tended with and removed by operations on portions of glass so 

 small that they are themselves almost microscopic objects, we 

 shall not be surprised that even a cautious philosopher and most 

 able manipulator like Dr. Wollaston should prescribe limits to 

 improvement. 



Fortunately for science, and especially for the departments 

 of animal and vegetable physiology, these predictions have 

 been shown to be unfounded. The last fifteen years have suf- 

 ficed to elevate the compound microscope from the condition 

 we have described to that of being the most important instru- 

 ment ever bestowed by art upon the investigator of nature. It 



