OF ARTS AND SCIENCES. 331 



" The following explanations will account for some of the arrange- 

 ments, the reasons for which are not obvious at first sight. It is very- 

 difficult to make wood slide on wood without adhesion and consequent 

 jerking. After many unsuccessful experiments, I found white-pine 

 would run smoothly over a surface covered with tinfoil. The unusual 

 length of the object-plate renders it much more manageable than the 

 ordinary ones. The arrangement of the lever not only gives extreme 

 accuracy and delicacy to the fine adjustment, but, the screw being in- 

 dependent of the rest of the instrument, the focus does not change 

 when the hand is lifted from it, as it does in many microscopes. The 

 wick-tube of the lamp is so placed that it can be brought close to the 

 object, and is at the same time at such a distance from the achro- 

 matic condenser as to give light enough without heating it to any ex- 

 tent which might injure the glasses. The wires by which the tube is 

 moved are a little above the level of the cradle, so as to admit of a 

 slight rocking motion of the tube. The delicacy of the coarse adjust- 

 ment is such, that the use of the fine adjustment may very often be 

 dispensed with. 



" This instrument cost between three and four dollars. I have been 

 so well pleased with its performance, that I have ordered one to be 

 made of brass and iron, with a hollow pillar instead of the tripod, with 

 several modifications, but the same general arrangements. Such an 

 instrument may cost about ten dollars, and would offer some advan- 

 tages over a carefully constructed one of the cheaper materials, which, 

 however, would do good service. I hope to have the opportunity of 

 showing a more nicely executed instrument, of the same general form 

 with this, at a future meeting of the Academy, and if it meets the ap- 

 probation of microscopists, I shall request a competent workman to 

 make them at the lowest rate he can afford, for those who are disposed 

 to try a new and somewhat peculiar piece of mechanism. 



" Many will at first object to the vertical stage, in the belief that 

 fluids cannot be conveniently examined in the position this requires. I 

 believe this objection is of little importance. Capillary attraction, 

 which holds mercury suspended in a fragment of thermometer-tube, is 

 surely enough to support any watery fluid and its contents between 

 two plates of glass, if the film of fluid is thin enough. Currents there 

 will often be, and currents there constantly are on a perfectly hori- 

 zontal stage, unless various precautions are taken, which I shall not 

 here stop to indicate. I have found no practical difficulty in examin- 



