68 



nation of these objects, and they had certainly turned out better than he had 

 expected. He thought the best results had been obtained by a spot lens, 

 thrown somewhat out of focus ; then the water-marking came out very 

 beautifully ; the stereoscopic effect also was very remarkable. 



Dr. Matthews said he conld testify to the extreme beauty of these curves, 

 and also to the stereoscopic effect, which seemed to be due to some illusion. 

 These figures appeared to him to throw some light upon the subject treated 

 of by Mr. Sorby, in one of his addresses, as to the ultimate limit of vision, 

 for, as the lines approached one part of the curve, they became so close that 

 it would be impossible to estimate their value unless their number had been 

 previously known. 



Mr. Ingpen said he should like to ask Mr. West what kind of diamond 

 point was used in drawing these figures, and also how it happened that the 

 crossings of the lines, though so fine, showed no signs of splitting. 



Mr. West could only say that he thought he had been rather fortunate in 

 fixing the diamond in a good position ; the point seemed to be rather blunt, 

 and, in shape, like that of a minie bullet. 



Mr. Ingpen enquired what kind of glass was used ? 



Mr. West said he had used the ordinary microscope slips. He believed 

 these were made of two sorts of glass, some being patent plate and others 

 flatted crown. His chief difficulty had been with the surface of the glass. 

 He did not think that the glass used for ruling micrometeis would answer 

 so well. 



Mr. Powell, in reply to a question from Mr. Ingpen, said that the glass 

 used for micrometers was the ordinary thin covering glass. 



Mr. Curties said there was a great difference between the cuts in the case 

 of a micrometer line and the lines drawn by Mr. West ; in the latter the 

 point seemed rather to slide along the surface than to scratch into it. 



Dr. Matthews said that, according to what he had seen of these lines, they 

 seemed to be more like shallow cuts than scratches, in which there had been 

 a removal of surface, so that if this were the case he should hardly expect 

 any dust to be produced. Every one who had made use of a glazier's 

 diamond was aware that there were two kinds of marks which could be 

 made with it, one being a true cut, and the other a scratch. 



Mr. Spencer said that whenever a cut was made there was no dust pro- 

 duced. Dust only was made when the attempt to cut was a failure. He had 

 often tried to get a cut with the black diamonds, but could never succeed. 

 The true cut was something physical and molecular, rather than mechanical. 



Mr. Ingpen said they must have a natural crystal of diamond to cut with ; 

 a point of a broken surface would be of little use. 



Dr. Matthews remarked that in the new steel roller for cutting glass the 

 curves of the edge were much the same as in the cutting diamond, and the 

 same effect on the gla^s was produced. There was no cut through but it 

 caused some kind of molecular disturbance. 



Mr. West said that he had found dust in the course of his experiments, 

 but he had not examined it. He had also tried to fill in the lines, but had not 

 yet been successful with either lamp-black and oil, carmine, or aniline; he 



