8 O N T H E C O N S T R U C T 1 O N A N D U S E O F 



aucl the effect of the position of the openings in the nionld for the entrance of the 

 molten metal. 



Effects of Heat. — Tlie preceding clianges hi glass when isolated appear very 

 simple, and their remedy, to keep the proper diameter perpendicidar, is so ohvious 

 that it may seem siu'prising that they shonld ha\(> given origin to any embarrassment. 

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

 practice they are 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 lieat : — 



A 15| inch mirror, which was giving at its centre of curvature a very fine image 

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



Fie. 4. 



Efl'ects of Heat on a Ketieuting Surface. 



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

 seconds an arc of light came out from tlie image as at //, and on putting the h-ft 

 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; (V to become some- 

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

 the focus the compk^mentary appearances, h, c, </, were found. 



By unsyrametrical warming still more remarkable forms emerged in succession, 

 some of which were more like certain nebuke 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 woidd ]ia\{' re- 

 (piired either re-grinding or jirolonged ])olishing 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 onlv show the image of a pin-hole as a 



Effects of Heat rendered permanent. , ,7T-i-r\-i • , i i--i 



lozenge (o, rig. o), Avith an image at each angle niside 

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



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



