330 PROCEEDINGS OF THE AMERICAN ACADEMY 



two inches long, with two small springs to hold the glass slide on which 

 the object is placed. The diaphragm is three inches in diameter, and 

 is let into the stage, so as to be close behind the object-plate without 

 touching it. 



" The lamp is of an oblong square form, three inches in length, one 

 and a half in width, one in depth, and fits snugly in a square box, with 

 two wires, each three inches long, projecting from its extremities. 

 The wick-tube, which is made small, to insure a bright flame, is close 

 to the edge at the middle of one side ; opposite to it is a slide, run- 

 ning up and down, and receiving an objective, by means of the com- 

 mon bayonet joint, to be used as an achromatic condenser. If the 

 lamp flickers, it is guarded by a piece of tin four inches in height, bent 

 to form three fourths of a circle three inches in diameter, and blackened 

 on the inside. Two bent pieces of wire, driven into the end of the 

 hed next the observer, serve to hold a hood or shade made of four 

 pieces of thin board covered with black velvet, the top and sides turn- 

 ing down so as to shut out the light, the central piece cut in such a 

 way as not to touch the tube or the cradle, which is its principal dif- 

 ference from that of Mr. Lister, as described by Quekett. 



" To use the instrument, the elbows are rested on the platform, when 

 all the preliminary arrangements are found close to the hands. The 

 coarse adjustment is made, the object brought into position, the light 

 arranged, with a precision that no machinery can surpass, because both 

 the arms and hands are perfectly steadied. If the diaphragm is wanted, 

 it may be easily reached with the fingers. Lastly, the fine adjustment 

 is made by dropping one hand to the screw, and twirling its head back 

 or forward with a single finger. If achromatic light is wanted, it is 

 always ready ; it is only necessary to turn the lamp half round, and 

 bring the objective so as to illuminate the object to the best advantage. 

 The intensity and obliquity both of common and achromatic light are 

 variable to any extent with the same facility, by moving the lamp back 

 and forward, or from side to side. None of the ordinary arrangements 

 of microscopes admit of using oblique achromatic light efficiently and 

 conveniently, if they allow it at all. 



" If it is required to use this microscope by daylight, a small prism 

 is placed directly before the hole in the stage, or the achromatic con- 

 denser, and turned until the proper illumination is obtained. To make 

 the light oblique, the disk carrying the tripod is revolved on the plat- 

 form. 



