CONCAVE MIRRORS. 441 



images not to be compared with the sharply-defined images of good 

 concave reflectors ; the lustre of their surface is also very soon tar- 

 nished. This latter defect is, however, easily remedied by making 

 a new mirror, while the mirrors thus produced are well adapted for 

 projecting a distinct image of a candle flame upon a screen. The 

 alloy is made of 29 parts of tin and 19 of lead ; it melts easily, 

 and if a clean surface of glass is pressed upon the molten liquid, 

 just when it becomes nearly solid again, a very bright impression 

 of the surface will be obtained. For making a concave mirror a 

 curved piece of glass, a so-called convex lens, is used ; in fact any 

 burning-glass will do for the purpose, but it is best to use a kind of 

 lens about 6 cm in diameter, mentioned in the next paragraph. This 

 lens is only slightly convex a very convex one produces obviously 

 very concave impressions, but they are nearly useless on account 

 of the distortion of the images produced. This distortion is always 

 great, and the image blurred, if the width of the mirror is more than 

 T Vth of the radius of curvature, and the concavity exceeds the p 1 ^ - 

 part of the width ; in fact, what has been stated above of concave 

 mirrors holds good strictly only for mirrors of which the con- 

 cavity is within the limits just mentioned. Mirrors which have 

 such a small curvature (and this is the only kind used for astrono- 

 mical purposes) require a -rather large space for exhibiting the 

 various kinds of images which they produce. It is therefore more 

 convenient to use lor experimental demonstrations mirrors of some- 

 what greater curvature. In our figures for illustrating the forma- 

 tion of images the curvature is always represented as greater than 

 it actually is, or otherwise the figures could not have been appor- 

 tioned to the size of the page. The actual curvature of our lens is 

 such that the centre is raised by about ^th f tae width of the 

 lens. The concave mirror will accordingly have a corresponding 

 depression of its centre. 



Prepare two square pieces of stout millboard, each side about 

 10 cm long, and out of the middle of one of them cut a circle having 

 its diameter a few millimetres shorter than that of the lens ; glue 

 both pieces flat upon each other, and they will form a shallow circular 

 mould for receiving the molten metal. The glue must be thoroughly 

 dry before the mould is used, and even then bubbles of vapour will 

 be driven out of the millboard by the hot metal. It is therefore 

 necessary to pour liquid metal once or twice into the mould, and 

 allow it to get cool in it, so as to have it thoroughly dried. 



A large cork glued to the lens serves as a handle. It should be 

 slightly hollowed out on the side where it is attached, so as to 

 require as little glue as possible a thick layer is difficult to dry, 

 and softens again when the glass is warm. 



