142 SELECTION AND TJSE 



vantage to be able to select that part which answers our pur- 

 pose most perfectly, and this plate affords peculiar facilities for 

 effecting this. 



Table. The table used for supporting the microscope 

 should be firm and substantial, so that all shake and vibration 

 maybe avoided. Those who 'use very high powers, and desire 

 to avoid vibration as much as possible, will find that a barrel or 

 box, filled with sand, and resting on three feet, makes the best 

 support. Some years ago, having some rather delicate investi- 

 gations to make, we constructed a table in this way, and found 

 the results very gratifying. Our table was arranged as follows: 

 a common barrel, cut down a little, and filled with sand, was 

 supported on three stout blocks nailed to the bottom. The 

 table proper was made of plank, nearly square, and it entirely 

 covered the top of the barrel. It was supported by a -{- shaped 

 piece of wood, which was fastened to the centre of the table, 

 and descended into the sand. With such a table, walking on 

 the floor, and the passage of heavy teams in the street, produce 

 no vibration, though, on an ordinary table, they render work 

 with high powers almost impossible. 



Where several persons wish to look through the same micro- 

 scope, it is very awkward if each one has to get up and go to 

 the instrument. At the same time it is of course impossible to 

 move the microscope without moving the arrangement for illu- 

 mination also. This difficulty has been avoided by means of 

 revolving tables, around which the observers sit, each one in 

 turn examining the object, as the microscope is passed round to 

 him. This is a very excellent, but a somewhat expensive ar- 

 rangement. The same end may be attained by placing the mi- 

 croscope, lamp, etc., on a smooth board of suitable size and 

 shape, and passing this board to each observer in turn. The 

 board, carrying microscope, lamp, etc., may be made to slide 

 quite easily, and if placed on three feet, it is tolerably steady. 

 Such a support, however, is not to be chosen where the micro- 

 scope is used for scientific investigations. 



