84 MICROSCOPIC ILLUSTRATIONS. 



confusedly, and produce a nebulous or indistinct impres- 

 sion of the magnified object. This defect is commonly 

 designated spherical aberration. Another obstruction to 

 perfect vision is occasioned by the effect produced upon 

 the light itself, when passing from one medium into 

 another of different density. Light (say common white 

 light) is a compound of coloured lights, which are not 

 equally refrangible ; that is, some of the colours are 

 more powerfully acted upon by a refracting body than 

 others ; so that a white ray, when it enters a lens, is more 

 or less broken up and separated into its coloured con- 

 stituents, which gives a semblance of colouring to the 

 object we are viewing. This defect is termed chromatic 

 aberration, or dispersion. 



These two great defects are to be obviated in con- 

 structing a microscope. Until within a few years, the 

 lens of a simple microscope consisted of a single piece of 

 glass made into a curved form, or a spherule. In order 

 to increase its magnifying power, i. e. in order to admit 

 an object being placed very near the lens, so^that it 

 might be viewed under a greater angle, Di Torre, 

 Leeuwenhoek, and others, constructed lenses of spherules 

 of exceeding minuteness some, it is recorded, with a 

 focal length of not more than l-700th or 1- 800th part of 

 an inch. This method of obtaining high magnifying 

 powers was attended with so much inconvenience in 

 practice that it counterbalanced all the advantages which 

 would otherwise have resulted from it. Whilst the 

 obstructions to vision just mentioned, viz. spherical and 



