306 



Mr. 0. W. Griffith on the 



The following table gives the diameter of apertures that 

 might be used with spheres of different radii. 





Diameter of aperture in cm. 



Radius of 

 sphere in cm. 



£ = •04 cm. 



£ = '05 cm. 



1 

 2 

 3 

 4 

 5 



•80 

 114 



1-39 

 1-60 

 1-80 



•90 



1-28 

 1-56 

 1-80 



200 



i 



Small glass flasks of 6 cm. diameter and thickness 

 *04-*05 cm., painted over with dull black varnish except an 

 aperture of about 1*5 cm. in diameter, seem to answer the 

 purpose very well, and good values of the index of refraction 

 of a liquid can be obtained by their use. It is essential that 

 the exposed parts of the flask should be spherical, and this 

 may be tested by measuring the radius of curvature at the 

 aperture with a spherometer and comparing it with the 

 distance between the two diametrically opposite apertures, as 

 measured with a vernier calipers. The parallel beam is 

 obtained by means of a collimator, and the position of the 

 focus determined by the optical bench eyepiece, or by means 

 of a travelling microscope, the source of light being the slit 

 of the collimator illuminated with radium light. By using a 

 short length of platinum wire heated with an electric current 

 as a source and a thermopile or bolometer as detecior, tho 

 method can be applied to obtain a rough estimate of the 

 refractive index for heat radiation of quartz in the form of a 

 sphere, or of a liquid (such as a solution of iodine in carbon 

 bisulphide), enclosed in a fused silica flask. 



It is beyond the scope of this note to discuss the general 

 question of spherical aberration, but the accompanying 

 diagrams are interesting in that they compare the aberrations 

 produced in a sphere and in the usual types of convergent 

 lenses. The effect of the aberration maybe represented in 

 two ways. If F is the power of the system, / its focal length, 

 E. the curvature of a surface and a the radius of the aperture, 

 and /uL the index of refraction, then the error produced by 



