ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. ' 131 



tioned. Sometimes the cutting crystal is lost after ruling a few lines, 

 but generally good results can be obtained after a constant service of 

 weeks, and even months. A crystal is lost either by being broken 

 off or by being worn out. When a crystal has been lost it need not 

 be concluded that the diamond needs sharpening. It is only necessary 

 to find a new crystal, an operation requiring patience rather than skill. 



It should be stated, that while this theory of individual cutting- 

 crystals seems to be the true one, I have never been able to detect 

 them by an examination with the Microscope. It is only by their 

 behaviour that their existence can be recognized. 



One of the most severe tests of the ruling qualities of a crystal 

 consists in producing, without fracture, heavy lines which cross each 

 other at a small angle of inclination, and which will receive graphite 

 without interruption of continuity at the intersection. Lines ruled 

 at right angles and forming small squares afford a better test than 

 parallel lines. In one plate presented the curved lines formed by 

 the intersection of straight lines are nearly perfect in form, and they 

 hold the graphite quite as well as the original lines. In another plate 

 I have attempted a representation of the nucleus of a comet. The 

 filling is not quite as perfect as in the other plate, but this is due to 

 the quality of the glass. Attention is called to the granular struc- 

 ture under a moderately high power. I have found rulings of this 

 form to be an excellent test of the quality of the glass required for 

 receiving the best lines. In general, the first filling of the lines is 

 the most perfect. One plate affords an illustration, exceedingly rare, 

 of lines which receive the lines equally well after repeated fillings. 

 Lines as fine as 50,000 to the inch very readily receive the graphite. 

 The limit beyond which it seems impossible to go may be placed 

 at about 100,000 to the inch. 



A few words may properly be added here with regard to the pro- 

 tection of ruled lines. When lines are formed by a true groove in 

 the glass, it is better that they should remain unprotected. But 

 when the lines are formed in the manner illustrated by the plates of 

 this series, the quality of the lines in the end is pretty sure to 

 deteriorate whenever there is an actual contact of the cover-glass 

 with the slide. I have made serious efforts to overcome this diffi- 

 culty, but with only partial success. Slides mounted with gutta- 

 percha rings generally remain in good condition for a long time, 

 especially if, after expelling the air as far as possible by heat, a ring 

 of white wax cements the rim of the cover-glass to the slide. But even 

 with this precaution there is no certainty of final preservation. If it 

 should be found that the brass slides of this series are convenient in 

 manipulation, their adoption can be recommended, since they entirely 

 obviate this difSculty. They are made in the following way: — A 

 hole having been made in the centre, a flange is left 1/200 in. 

 in thickness. The cover-glass is then cemented to the surface 

 of the brass, and the rulings are made on the under side. The 

 protection is made by dropping upon the ledge of brass a rather thick 

 circle of cover-glass, which is held in position by a circular brass 

 wire. 



K 2 



