302 



horizontal wires in the two microscopes. As care was taken, that 

 the moveable micrometer wires were parallel to the division lines, an 

 influence of a small deviation of the point npon which the micros- 

 copes are directed need not be feared. 



2. Microscopes. At our request the microscopes were so con- 

 structed, that on the objective-side the course of the rajs is tele- 

 centric ; they have a 30-fold magnification, and the illumination of 

 the division lines is not sideways but central, by means of a prism 

 with total reflection, which is placed behind the objective in the 

 tube of the microscope, and occupies half of the field. The light 

 from a small electric lamp falls through an opening in the tube of 

 the microscope upon the dull face of the prism, is then reflected by 

 the prism verlically downwards through the objective, falls upon the 

 reflecting surface of the metre or the measuring bar, and is thus 

 reflected vertically back into the microscope. With an electric lamp 

 of a few candles the illumination was excellent, and the division 

 lines were seen as very fine black lines. 



3. Temperature. The exact determination of the temperatures of 

 the metre and the measuring bar is a matter of great importance. 

 In order to make the changes of temperature as small as possible 

 we endeavoured, in the first place, to keep the temperature of the 

 room as constant as possible. For this pui-pose the windows were 

 covered with thick curtains, and the central heating was shut off. As 

 only a small amount of heat was convened through the floor and 

 walls, the temperature did not change much, and only rose a little 

 from the presence of the observers, and the burning of a few gas lamps. 



All tlie metal parts of the comparator were shut off from the 

 outside air by wood and other badly conducting material, outside 

 which only the eye-pieces of the microscopes protruded ; the measur- 

 ing bar was moreover entirely enclosed in a thick aluminium case 

 and the metre in a brass box, in which there were only small 

 openings for the reading of the division lines and the thermometers. 

 The protection of the metre from the radiating heat was less effective 

 than that of the measuring rod, so that in half of the measurements, 

 during which the observer was on the side of the comparator nearest 

 to the metre, it was found advisable to cover the outside wall of 

 the comparator with a layer of badly conducting material, which 

 gave a greater constancy of temperature. 



For the determination of the temperature of the metre and mea- 

 suring bar, upon the horizontal faces on which the division lines are 

 drawn a thermometer A was laid upon the metre, and on the measur- 



