152 Micro-ruling on Glass and Steel. 



but to the microscopist it is instructive from the various perspective 

 appearances it presents. From the principle on which it is ruled 

 the lines composing each ray proceed for some distance before 

 coming in contact with the lines belonging to adjacent rays, as 

 shown in Fig. 1, PI. XCVII. Each ray, at its peripheral end, com- 

 mences with what looks like a sharply-cut dot, the first impact of 

 the diamond point with the glass, and the lines composing each ray 

 gradually diminish in length, giving the wedge-shape shown in the 

 figure. When the star is viewed with a low power (say 3"), with 

 dark-ground illumination, the optical effect of the rows of bright 

 dots, with which the lines commence, is to suggest that each ray 

 stands up from the general plane of the glass, that all look like a 

 number of spokes of a fan placed, more or less, vertically on the 

 table. A higher power (frds) makes this more striking, as in 

 Fig. 2. The position of each ray, with reference to the angle 

 at which a light strikes it, determines how much it slopes either 

 right or left of the vertical plane, and one or two rays will 

 appear nearly in that plane. Keeping the illuminating apparatus 

 stationary, and revolving the stage, causes the apparent slope of 

 each ray to vary, and if any pair of rays be selected for particular 

 observation, they will be seen to undergo curious apparent changes 

 of position. At one point of the stage rotation it will appear as 

 if the eye beheld the outside of one ray and the inside of its neigh- 

 bour, while at another point the appearance will be reversed, and it 

 will look as if the outside of the first and the inside of the second 

 had come into view. If the object is then moved so that the wedge- 

 shaped ends of the rays are thrown out of field, the eye is somewhat 

 tempted to consider adjacent rays as not quite in the same plane, but 

 the striking illusion just described has disappeared. 



After proceeding separately for some distance, the rays com- 

 mence their contact, and the intersections of the several lines com- 

 posing them produce a secondary star, with finely-pointed rays, 

 gradually broadening towards their bases. It is easy to illuminate 

 these secondary rays so as to make this star appear in a plane 

 higher than that of the primary star, and to give an appearance of 

 solidity to each ray. 



Near the centre of the star, where the rays meet, the aspect is 

 beautifully watered and the lustre is silvery, or delicately iridescent, 

 according to the illumination. In the centre is a clear space, and 

 this has the aspect of a deep hole, an appearance much assisted by 

 the curvature of the lines as they come to the point. 



The appearance of concentric scorings seen on the secondary 

 star arises from intersections. It is striking with powers under 

 ■| inch, but with ^ inch and upwards they grow fainter, and dis- 

 appear in a conflict of cuts. 



Not to enlarge further on this particular star, it will be seen 



