348 THE MICROSCOPE. 



contrivance the tube, and consequently the lens (called the object ; glass) 

 attached to it, may he adjusted and maintained at any desired distance 

 from the object. The object-glass transmits into the tube an enlarged 

 inverted image, which, by regulating the distance between the glass 

 and the object, we can bring with accuracy to the upper end of the 

 tube. Here it is viewed through a moderately magnifying lens (the 

 eye-glass) to which the eye is directly applied. Instead of a single 

 lens as eye-glass, the practice soon obtained of using a combination of 

 two, arranged at something less than their focal distance from one 

 another, which, being set in a brass cylinder, might be inclosed in the 

 tube of the microscope. The lower and weaker of these two lenses 

 causes the rays of the image transmitted through the object-glass some- 

 what to converge ; it diminishes the image, but makes it brighter and 

 sharper. This image is once more enlarged by the upper lens of the 

 eye-glass. In the tube of the microscope and eye-glass perforated dia- 

 phragms are appropriately disposed, which exclude from the eye rays 

 passing through the rim of the differently refracting lenses. 



It was in this form that the second decennium of our century found 

 the compound microscope. It was a very imperfect instrument. Its 

 sole advantage over strongly magnifying single microscopes, over 

 lenses of short focal distance to which the eye was immediately applied, 

 consisted in a greater convenience of handling ; an advantage more 

 than counterbalanced by its lower optical efficiency. All the faults of 

 single lenses in regard to the magnified images produced were still 

 conspicuous in proportion to the enlargement. The colored bordering 

 of the images still proved very annoying, and the narrowing of the 

 aperture, occasioned by the diaphragms which intercepted the side 

 rays, robbed the images of a large share of light. Besides, only weak 

 object-glasses could be employed. Hence it was that several of the 

 most distinguished microscopists continue to avail themselves prefera- 

 bly or exclusively of single microscopes. 



Up to the time of the modern great improvements in compound mi- 

 croscopes, the most important observations and discoveries had been 

 made with single microscopes, from the researches of Leuwenhoek, 

 which led the way (first decennium of the eighteenth century) to those 

 of Robert Brown, of whose striking discoveries we shall here only men- 

 tion that which led him to detect a certain independent and oscillatory 

 motion of small portions of organic and unorganic bodies floating in 

 liquids. 



The first step to these improvements was the elimination of the col- 

 ored circles fringing the images of the microscope. The degree in 

 which different transparent bodies refract the rays of light does not in 

 all cases bear an equal proportion to that in which, in refracting, they 

 separate white light into the colors of the spectrum. While crown- 

 glass, for instance, strongly refracts the ray, it separates it into the 

 different colors only in a moderate degree. Flint-glass, on the other 

 hand, effects the prismatic dispersion in a much greater degree, while 

 the refraction of the rays is not greater than in crown-glass. The 

 optician has hence a means of preparing compound lenses, constructed 

 of concave and convex lenses, which transmit the light almost without 

 prismatic dispersion. If a convex crown-glass lens be joined with a 



