77-1 Prof. R.W. Wood on (he 



bide, for example, sloping- at an angle of 12° with the original 

 surface, the other at 48°. These angles are subsequently 

 determined with a small spectrometer or by simply mounting 

 the gratings on a graduated circle, and observing the reflexion 

 of a lamp-flame in them. In the best ones no trace of the 

 central image can be seen, which is what we should expect 

 if the edges of the grooves were sharp and none of the 

 original plane surface remained. The sum of the two angles 

 of slope did not always add up to 60°, as they should do 

 if the ruling had been done with a 120° point. This is 

 probably due to the circumstance that the edges of the 

 carborundum crystals are usually bevelled as shown in fig. 1. 

 1 have not made a study of the angles at which these small 

 planes meet, but it seems likely that with certain crystals we 

 may have a ruling point the edges of which meet at an angle 

 larger than 120°. 



Of the eight gratings which I have measured thus far, 

 the angles of the edges and their sum are shown in the 

 following Tabhi : — 



11 



20* 



5 



22 



29 



11 



12 



17 



43 



49 



27' 



5 



30 



18 



44 



46 



37 



11 



HO 48 52 47 55 58 54 54 



Tn addition to knowing the angle of slope of the two sides 

 of the groove it is necessary to determine whether they 

 make a sharp angle, i.e. whether they meet in a knife-edge 

 at the top, or whether there is some of the original surface 

 remaining between them, or a ridge of more or less roughed 

 surface due to the squeezing up of metal by the compression 

 resulting from the action ot the carborundum crystal. 



Some difficulty was found in interpreting the appearance 

 of the surface under the microscope until the following 

 method, which gave beautiful results, was tried. Two electric 

 lamps were placed just above the stage of the microscope, to 

 the right and left of the tube, in such positions that the 

 edges of the grooves reflected light vertically into the 

 objective. A red glass was placed in front of one lamp, and 

 a green glass in front of the other. The edges of the grooves 

 appeared brilliantly illuminated in complementary colours, 

 with no dark region, if they met at the top, but if not, each 

 pair of red-green strips was separated from the neighbouring- 

 pair by a dark line, due to the fact that the level surface 

 between the grooves was not at the proper angle to reflect 

 light from either lamp into the objective. The appearance 

 of the gratings illuminated in this way was similar to that 



