28 ON DIFFRACTION SPECTRA IN THE MICROSCOPE. 



of crossed lines, of which one group is made up of lines crossing 

 each other at an angle of 60** j whilst the other group consists of 

 two sets of lines crossed at an angle of 90®. Each group is thus 

 made up. — On the silvered surfaces of two thin pieces of glass are 

 ruled two sets of parallel lines beside each other. One of these 

 sets has a distance between the lines of '0073 mm., the other of 

 •01 1 mm., having thus a relation to each other bf 3:2. By laying 

 the silvered surfaces of the two pieces of glass together, and 

 inclining them at 60° to each other, three sets of lattice crossings 

 or gratings result. For the other group, at an inclination of 90**, 

 ^corresponding gratings are obtained. (PI. III., Figs. 10 and 11.) 



Illumination : — 

 An ordinary petroleum lamp is used with concave mirror. 

 When experiments are made on this simple group of parallel lines, 

 the lamp must be brought up pretty close and the flame turned 

 with its edge towards the instrument. For the experiments on the 

 crossed groups of lines the flame is turned with its hroad side to 

 the mirror, and a little further off than in the other trials : the plane 

 surface of the mirror is now used. 



Experiment I. — 



Having fixed the card diaphragm (see pi. II., fig. i. a, with three 

 slits,) on the adapter and screwed on the {a.a^ objective, focus the 

 microscope on the central group of parallel lines on the object 

 plate, using concave mirror and the flame of a common petroleum 

 lamp placed edgeways directly in front of the mirror (at 4 or ^ 

 inches distance) and at a level above it, suited to the inclination of 

 the body of the instrument. Now remove the eyepiece, and turn 

 the diaphragm until the three slits in it are situate at right angles 

 with the group of parallel lines. (PI. II., fig 2.) As the group 

 consists of two sets of lines whose interspaces are as 2:1, the 

 diffraction bands produced by each will have a different rate of 

 dispersion. This will be best seen by first observing each set of 

 lines separately. If, then, the set of lines ruled with widest 



