328 



PROCEEDINGS OF THE AMERICAN ACADEMY 



model every thing is kept in its place by gravity, except the object, which 

 is very easily supported and adjusted without any particular mechanism. 

 The moment ihejixed horizontal position is given up, the simplicity of 

 the instrument is gone. Thus the second principle, dispensing with 

 machinery, is carried out by adhering to the first. To fulfil the third 

 condition, that of employing the cheapest materials, it was necessary 

 to make use of various substances for the different parts of the instru- 

 ment. A piece of leaden pipe was fitted to hold the eye-glass and 

 objectives ; the latter being adapted by the bayonet joint of Chevalier. 

 Its weight was of advantage in keeping it in its place. The rest of 

 the instrument was made of wood, except the tripod, which was of 

 iron, and the lever, diaphragm, screw, and springs, which were of 

 brass. All the details will be described in connection with the figure. 

 " The following considerations determined some of the principal 

 dimensions. When a person sits before a common table, the eye, 

 when directed straight forward, is about eighteen or nineteen inches 

 above it. This determines the height of the eye-piece and tube. To 

 bring the hands to the height of the eye, with the elbows resting, the 

 elbows must be raised about three inches above the table. This deter- 

 mines the height of the platform on which the whole rests. Eighteen 

 inches will give width enough for the separation of the elbows. The 

 tube being about eight inches long, which is a common length for mi- 

 croscopic bodies, the pieces on which it rests may be each of them an 



inch shorter. The common length 

 of the English glass slides being 

 three inches, the stage must be 

 five inches wide, to admit of their 

 moving about freely. From these 

 dimensions several of the others 

 are naturally derived. 



On looking at the woodcut, it 

 will be seen that the whole instru- 

 ment is supported on an oblong 

 square platform, the dimensions of 

 which are 18 X 10 X 3. On this 

 is placed a revolving disk, eight 

 inches in diameter, fitting upon a 

 pin in the centre of the frame. 

 A tripod, the legs of which are 



