32 Professor Ayr Ion [Jan. 24, 



mirrors that the apparent reflection of the back is observed. The 

 mirror is placed face uppermost flat on a wooden board, and then 

 scraped or rather scratched with a rounded iron rod about half an 

 inch in diameter and a foot long, called a megeho, " distorting rod," 

 so that a series of parallel scratches is produced, which causes the 

 face of the mirror to become convex in the direction at right angles to 

 the scratches, but to remain straight parallel to the scratches ; in fact 

 it becomes very slightly cylindrical, the axis of the cylinder being 

 parallel to the scratches. This effect is very clearly seen by applying 

 a straight-edge in different ways to the face of an unpolished mirror 

 which has received a single set of scratches only. A series of 

 scratches is next made with the megeho in a direction of right angles 

 to the former, a third set intermediate between the two former, and so 

 on, the mirror each time becoming slightly cylindrical, the axis of the 

 cylinder in each case being parallel to the line of scratches, so that 

 eventually the mirror becomes gfenerally convex. Some workmen 

 prefer to make the scratches with the megeho in the form of small 

 spirals, others in the form of large spirals ; but the general principle 

 of the method employed with their mirrors appears to be always the 

 same, — the face of the mirror is scratched with a blunted piece of 

 iron, and becomes slightly convex, the back, therefore, becoming 

 concave. 



[Some mirrors were here exhibited : one with its surface flat, 

 although somewhat rough, just as it came from the mould after 

 casting ; a second that had received one set of parallel scratches with 

 the megeho, and which by means of a straight-edge was shown to be 

 slightly cylindrical ; and a third, on the face of which the operation 

 of scratching had been completed, and which was, therefore, slightly 

 convex.] 



After the operation with the " distorting rod " the mirror is very 

 slightly scraped with a hand scraping-tool to remove the scratches 

 and to cause the face to present a smooth surface for the subsequent 

 p*olishing. 



In the case of thick mirrors the convexity is first made by cutting 

 with a knife, and the " distorting rod " applied afterwards. But in 

 connection with this cutting process of thick mirrors there is one very 

 interesting point. If the maker finds on ai:)plying from time to time 

 the face of the mirror to a hard clay concave pattern, and turning it 

 round under a little pressure, that a portion of the surface has not 

 been in contact with the pattern, in other words, that he has cut away 

 this portion too much, then he rubs this sjjot round and round with 

 the megeho until he has restored the required degree of convexity. 

 Here again, then, scratching on the surface produces convexity. 



Now, why does the scraping of the " distorting rod " across the 

 face of the mirror leave it convex ? During the operation it is visibly 

 concave. The metal must receive then a kind of " buckle," and sjDriug 

 back again so as to become convex when the pressure of the rod is 

 removed. It might in such a case reasonably be expected that the 



