AMERICAN INSTITUTE. 441 



ceedingly minute objects. It is furnished with object lenses and eye-glasses 

 which may be used in combination or singly. When it magnifies an object 

 five hundred times, it becomes very distinct, but if four thousand times, 

 the outlines are not perfectly preserved. The mirror consists of a frame 

 of brass, in which two silvered glasses are set, one plane and the other 

 concave, which should not be less than two inches in diameter. In the for- 

 mer the light is reflected in converging rays ; the latter in parallel rays. 



Very little is generally known of the management of modern compound 

 microscopes ; consequently many of those who have good ones cannot use 

 them advantageously, finding great difficulty generally in overcoming the 

 spherical aberrations of the light's rays. Even manufacturers sometimes 

 err in the principles of their construction, and find to their surprise that 

 the glass disperses the rays, and produces prismatic colors. The coloring 

 of images may be easily prevented by a proper achromatic lens ; and to 

 overcome spherical aberration, the object glass must be arranged so as to 

 neutralize the negative and positive rays, of which spherical aberration 

 consists. The microscope manufacturer, when arranging his instrument 

 for illuminating objects, and requires a condenser of light, should imitate 

 nature, as there is no better condenser than a white cloud between the earth 

 and the sun. He may use a white disc of plaster for this purpose. A 

 mirror is generally made use of, but is bad, because it reflects the rays 

 through the object, confuses the vision, and prevents a good definition 

 through the medium of a glare of light. The construction of a magnify- 

 ing power for a compound microscope is complicated, because several lenses 

 must necessarily be employed to form one magnifier ; and not only that, 

 two difl'erent kinds of glass must be used for one lens of such a magnifier ; 

 and perhaps there is no single problem throughout the range of this noble 

 science that has occupied the mind of the learned throughout all lands as 

 that of achromatism, and achromatic combinations ; all the first attempts 

 were unsuccessful, and even within the last twenty-five years, such great 

 men as Wallaston and Biot actually predicted that the compound micro- 

 scope would never be superior to the simple when supplied with doublets. 

 Notwithstanding the opinion of these great philosophers, within the last 

 few years the compound microscope has become the most important instru- 

 ment ever bestowed by science upon nature's investigator, man. 



With it nature has become widely explored ; the gushing brook, rippling 

 stream, and quiet lake, have exposed to its magnifying power their congre- 

 gated myriads of insect life ; before its invention, when we examined one 

 species of water it was red, another craiige, another green, but we knew not 

 the cause ; we now know the colors are formed by living bodies, and that 

 they swarm throughout all seas, and all lands ; mud brought from the bot- 

 tom of the ocean, by the lead, full fifteen hundred feet in depth, was found 

 full to repletion of organic life. The air we breathe abounds not only with 

 living animalcules, but their eggs, raised by evapoi-ation from the surface 

 of the earth, and borne unseen by the naked eye into the atmosphere, ready 

 to develope life when circumstances are favorable, which sometimes does 



