1876.] On a new Refractometer. 393 



IV. e ' On a new Refractometer for measuring the Mean Refractive 

 Index of Plates of Glass and Lenses by the employment of 

 Newton's Rings/' By G. W. Royston-Pigott, M.A., M.D. 

 Cantab.,E.R.S. j> F.R.A.S.,F.C.P.S. Received March 19, 1876. 



The instrument depends upon two principles : — 



I. The distance through which an image is displaced by refraction 



through a plate. 

 II. The exquisite sensitiveness of contact-films f ormiug the various 

 orders of Newton's rings and the central black spot. 

 I. It is well known that when the index of refractiou is 1*500 or f, 

 an image is formed by a plate haviug parallel sides at a distance 

 nearer to the eye of the observer equal to two thirds of the thickness of 

 the plate. Indeed if t be the thickness and fi the refractive index, the 

 displacement is generally 



t 



If, therefore, an instrument could be devised which would with great 

 accuracy measure the thickness of the refracting plate, and also at the 

 same time the distance by which the image of points on its surface was 

 displaced inwards by refraction, data could be obtained for determining 

 the value of p. 



The instrument has assumed its present form after many construc- 

 tions and reconstructions. I was led to consider this method of finding 

 the refraction of glass by frequent accidents happening while using the 

 fiftieth-of-an-inch objective with the microscope, which by pressure 

 destroyed or cracked the thin glass covers generally applied to protect 

 the objects or " slides." Now, so to speak, the observer always in such 

 a case really examines the elevated image of the object, raised about 

 two thirds of the thickness of this cover. By knowing, therefore, the 

 refraction of the glass cover and its thickness, such accidents, so irre- 

 parable in many valuable objects, might be avoided. Means were sought 

 to determine the index of refraction of such covers, frequently varying 

 from the hundredth to the thousandth of an inch thick, an extra thick- 

 ness sometimes being destructive to the most valuable objective. 



Hitherto the method of finding the refractive index has been by the use 

 of prisms made of the material in question, and employed in the form of 

 a spectroscope. 



As an example of the power of the instrument, some fiint glass, nearly 

 half an inch thick, marked B, gave on three trials 



/u = 1-6626 

 1*6626 

 1-6621 



Mean =1-6624 



VOL. XXTV. 2 G 



