certain Dilute Aqueous Solutions. 379 



5. They must be parallel to the beam of light. 



6. The fork F 2 must be parallel to the ways of the engine. 

 This last is easily accomplished by using a plumb-line hung 

 from the rod R. 



1. The lenses L x and L 2 are placed in a tin tube against 

 diaphragms perpendicular to the axis of the tube. This makes 

 their axes practically coincide. The lamp S is then adjusted 

 so that the light (from the filament that is to be used) after 

 passing through Lj covers the centre of L 2 . Then the fila- 

 ment and the axes of L t and L 2 are in a straight line. 



2. The lens L x is moved until the light is focussed on S 2 , 

 and then L 2 is moved until, on reflecting the beam of light 

 back along its original path, it appears to converge to the 

 hole in S 2 through which it originally came. The position of 

 L 2 is then adjusted until the light is as nearly parallel as one 

 can detect with a small telescope focussed for infinity. 



3. The entire system L 2 , L 1} and S is then inclined by 

 le veiling-screws until the beam of light is as nearly horizontal 

 as can be detected by means of a water-level two metres long. 



4. The engine is placed on a slab of slate mounted on 

 levelling-screws and at the desired height. The horizontality 

 of the ways is tested by means of a delicate spirit-level. 



5. If two strips of -paper are pasted on M l5 so that their 

 edges touch opposite sides of the part of M x that is covered 

 by the beam of parallel light ; and if the engine is moved 

 until the light remains on this spot while the carriage is 

 moved over the entire length of the ways (about 50 cm.), we 

 may be sure that the ways are quite approximately parallel 

 to the beam of parallel light. 



If water is now placed in the tray, if the mirrors and the 

 telescope are turned so that the light is thrown down the tube 

 of the telescope, and if cross-threads are stretched across L 2 , 

 a dark cross will be seen in the field of the telescope when it 

 is focussed for a point about one metre below the surface of 

 the water (which is the focal length used throughout the 

 work). The cross-threads of the telescope are made to coin- 

 cide with this cross, and the carriage is moved along the ways. 



If the ways are accurate, and if the other adjustments have 

 been well made, these crosses will coincide throughout the 

 length of the screw. But, owing to slight irregularities in 

 the ways, it is found that the crosses are relatively very 

 slightly shifted, though from scale-reading 15 to scale- 

 reading 29 they remained coincident, except from 22'6 to 

 21*3 where one bearing of the carriage rests upon a small 

 dent in one of the ways. Hence all settings were taken 

 between 16*5 and 22, and between 25 and 27. In these 



