462 



SUMMARY OF CUERENT RESEARCHES RELATING TO 



Fig. 74. 



the so-called herapathite require great manipulative skill for their 

 production. If these could be readily obtained of sufficient size, they 

 would be invaluable as analysers. 



This opinion is supported by the existence of an inconvenience 

 which attends every form of analysing prism. It is frequently, and 

 especially in projecting apparatus, required to be placed at the focus 

 of a system of lenses, so that the rays may cross in the interior of the 

 prism. This is an unfavourable position for a prismatic analyser, 

 and in the case of a powerful beam of light, such as that from the 

 electric arc, the crossing of the rays within the prism is not un- 

 attended with danger to the cementing substance, and to the surfaces 

 in contact with it. 



Abbe's Analysing Eye-piece.— This (fig. 74), devised by Prof, 

 Abbe, consists of a Huyghenian eye-piece with 

 a doubly refracting prism P (a calc-spar prism 

 achromatized by two suitable glass prisms) 

 inserted between the eye-lens O and field- 

 lens C, and over the diaphragm at B. The 

 rays polarized parallel to the refracting edge 

 pass through the prisms without deviation, 

 whilst those polarized at right angles are 

 strongly deflected, and are stopped off by a 

 diaphragm over the eye-lens. The field of view 

 remains undiminished. 



Measurement of the Curvature of Lenses.* 

 — With very small lenses the spherometer can- 

 not be used, and Prof. E. B. Clifton's method 

 is based on the Newton's rings formed between 

 the lens and a plane surface, or a curved surface 

 of known radius. From the wave-length of the 

 light employed in observing and the diameter 

 of a ring the radius of curvature can be deter- 

 mined. He places the lens on a plane or curved 

 surface under a Microscope, and lights it by the sodium flame 

 — wave-length 5892 X 10"''' — measures the approximate diameters of 

 two rings a distance apart (in practice the tenth and twentieth rings 

 are found convenient), takes the difference of their squares, and 

 divides it by the wave-length and the number of rings in the gap 

 between to find the radius of the lens. The formula is : — 



P^mX = {x\+^ - x\) 



where x,„ -\- „ and a;„ are the diameters of the nth. and (m + n)th. rings ; 

 X is the wave-length of the light, and p^ the radius of curvature of 

 the lens. The method with proper care gives accurate results. Prof. 

 Clifton has also used it to determine the refractive index of liquids in 



' Nature,' xxix. (1883) p. 143. 



