8 ON THE CONSTRUCTION AND USE OF 



and the effect of the position of the openings in the mould for the entrance of the 

 molten metal. 



Effects of Heat. — The preceding changes in glass when isolated appear very 

 simple, and their remedy, to keep the proper diameter perpendicular, is so obvious 

 that it may seem surprising that they should have given origin to any embarrassment. 

 In fact it is now desirable to have a disk in which they are well marked. But in 

 practice they arc complicated in the most trying manner with variations produced 

 by heat pervading the various parts of the glass unequally. The following case 

 illustrates the effects of heat: — 



A ISg inch mirror, which was giving at its centre of curvature a very fine image 

 (rt, Fig. 4) of an illuminated pin-hole, was heated at the edge by placing the right 



Effects of Heat ou a Reflecting Surface. 



hand on the back of the mirror, at one end of the horizontal diameter. In a few 

 seconds an arc of light came out from the image as at h' , and on putting the left 

 hand on the other extremity of the same diameter the appearance c' was that of 

 two arcs of light crossing each other, and having an image at each intersection. 

 The mirror did not recover its original condition in ten minutes. Another person 

 on a subsequent occasion touching the ends of the perpendicular diameter at the 

 same time that the horizontal were warmed, caused the image d' to become some- 

 what like two of c', put at right angles to each other. A little distance outside 

 the focus the complementary appearances, h, c, d, were found. 



By misymmetrical warming still more remarkable forms emerged in succession, 

 some of which were more like certain nebidae with their milky light, than any 

 regular geometrical figure. 



If the glass had, after one of these experiments, been immediately put on the 

 polishing machine and re-polished, the changes in sur- 

 face would to a certain extent have become permanent, 

 as in Chinese specula, and the mirror would have re- 

 quired either re-grinding or prolonged polishing to get 

 rid of them. This occurred unfortunately very fre- 

 quently in the earlier stages of this series of experi- 

 ments, and gave origin on one occasion to a surface 

 which could only show the image of a pin-hole as a 

 lozenge (6, Fig. 5), with an image at each angle inside 

 the focus, and as an image a with four Avings outside 



But it must not be supposed that such apparent causes as these are required to 



Effects (if Heat reudereJ perinauent. 



