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II. Measurement of Curvature and Refractive Index. By 

 C. Vernon Boys, A.R.S.M., Demonstrator of Physics at 

 the Normal School of Science, South Kensington*. 

 [Plate I.] 



OF the methods best known for measuring the curvature 

 of surfaces, that depending on the spherometer is both 

 accurate and convenient in the case of surfaces of sufficient 

 diameter and where the curvature is not too small. The 

 reflection-test, depending on the observation with a telescope 

 of two images projected on a scale, is certainly not convenient, 

 nor is it capable of giving very accurate results. "While 

 endeavouring to find some more satisfactory way of exa- 

 mining the curvature of the surfaces of lenses that would be 

 both accurate and simple, I discovered the method which I 

 am about to describe. Though, from its great simplicity, I 

 can hardly expect it to be new to every one, yet I have never 

 heard of its being employed, nor do those to whom I have 

 shown it remember to have seen it before. Therefore, even if 

 it should be shown that this method is not new, it is certainly 

 so little known that I think it worthy of attention. 



The centre-of-curvature test known as Foucault's test, 

 which is used to examine the figure of the mirrors of re- 

 flecting telescopes, gives, perhaps, the most delicate means of 

 examining form that exists. By its means the expansion by 

 heat of a portion of the surface produced by touching it with 

 the finger is rendered evident, as an apparent mountain 

 standing out of the glass, which takes from five to fifteen 

 minutes to disappear ; and the warm air leaving a hand held 

 between the centre of curvature and the surface has the 

 appearance of flames of fire. In each of these cases no other 

 system of observation could show in so striking and conspi- 

 cuous a manner effects depending on so slight a cause. 



The examination at the centre of curvature is carried out in 

 this way: — The mirror is placed in a convenient support so 

 that its surface is vertical; and in front of it is placed a lamp 

 with an opaque chimney, through which some pin-holes have 

 been made. If one of these holes is near the centre of cur- 

 vature, the light leaving it and reflected by the surface is 

 brought to a focus on the other side of the centre. This focus 

 is then found ; and the lamp is moved till the focus is as near 

 the chimney as will allow of its observation. A piece of thin 

 sheet metal, with a straight edge, is then placed so that it 

 may be moved to or from the mirror or laterally. Now, 

 if the edge of this is sufficiently on one side to let the light 

 * Communicated by the Author. 



