4:40 TRANSACTIONS OF THE 



seen by all the room ; an ignited jet of oxygen and hydrogen gases npou 

 carbonate of lime, as well as light produced by the galvanic battery, has 

 been used advantageously in place of the solar rays. 



The Polytechnic Institution, in England, have a new oxyhydrogen mi- 

 croscope, made by Carey, consisting of six powers, ranging from one hun- 

 dred to seventy-four millions of times. The second power magnifies the 

 wings of the locust to twenty-six feet in length ; the fourth power magni- 

 fies the sting of the honey bee to twenty-five feet ; by the sixth power the 

 eye of the house fly is magnified to such an extent, that each lens appears 

 to be 12^ inches in diameter, and the human hair 17 inches in diameter. 



If day light is made use of as the illuminating agent, your instrument 

 must be placed so as not to be exposed to the direct rays of the sun. A 

 white cloud opposite that great luminary, is the point from which the most 

 intense light is given ofi". A dark cloud in the same position is one from 

 which the fewest rays proceed. 



You may always estimate the magnifying power of either simple or com- 

 pound lenses, by comparing any object of known size, that can be perfectly 

 seen ; this distance is called the standard of distinct vision, and with it 

 you may compare all the magnifying powers of your lenses. Opticians 

 now adopt ten inches as the standard. And this decimal is certainly a 

 useful number, because with it as a standard, the magnifying power of 

 lenses, of any focal length, can be easily determined. If the lens under 

 examination be of one inch focus, we have only to add a cipher to the de- 

 nominator of the fraction, which expresses the focal length of the lens, and 

 the result will necessarily be the magnifying power. Thus if the lens be 

 half an inch focal length, the magnifying power will be twenty diameters. 



Every object desired to be examined by the microscope belong to one of 

 two classes, the opaque or transparent, and the methods of illumination 

 difi"er for each class. In the former the light is reflected upon them by 

 means of a convex mirror, whilst in the latter it is condensed upon them by 

 a lens. 



THE SINGLE MICROSCOPE. 



In this instrument an object becomes visible by rays of light emanating 

 on its surface, which are received by the eye and concentrated upon the 

 retina, thus forming a perfect image on the inner posterior surface of the- 

 eye, provided it is at the focal distance of the lens. A single microscope 

 may be made by piercing a circular hole in a piece of metal, and intro- 

 ducing into it a drop of water, which immediately assumes a spherical 

 form on both sides of the metal. Kock crystal makes the best microscopic 

 lenses. Flint glass is not fitted for them, because it possesses too great a 

 dispersive power. The crystalline lenses of small fish give a perfect image- 

 of minute objects. 



COMPOUND MICROSCOPE. 



This instrument is formed by combining two converging lenses, whose 

 axis are placed in the same line, and possesses the power of developing ex- 



