398 On a new Refractometer. [Mar. 23, 



T = 0*0163 

 Initial reading =0*0045 



Thickness =0-0118 



A = 0*0042 elevation o£ image. 



v =0-0076 distance of image from upper surface. 



Therefore ^=-=^^==1.55 nearly. 



r v 0-0076 J 



I obtained from Mr. Browning several square pieces of flint glass of 

 varying density and colour, and found by a series of measurements — 



Value of fi for mean rays. 



A. Clear white flint 1*537 



B. Yellow and heavy 1-6626 



C. Yellowish and very heavy 1-723 



D. Strong yellow, and the heaviest of them all . . 1-7555 



2. By reflected Light. — Far more interesting are the phenomeua 

 developed by reflected light, throwing the light laterally upon the 

 inclined transparent plane within the microscope, the object-glass of 

 which acts as its own condenser. The most beautiful colours are deve- 

 loped on contact, either with a film of air or kerosine. 



The central spot of final contact is of a grey-black, surrounded with 

 its well-known succession of Newtonian rings of great beauty and per- 

 fection, flashing through numerous chauges of colour for each thickness 

 of film varied by the micrometer-screw. 



The instant of the formation of the black spot film (thickness 0-000005 

 inch) determines the place of contact, i. e. the fiducial point of zero, 

 with a precision scarcely equalled by any other known method of linear 

 measurement by optical means. It can be readily obtained with great 

 accuracy. This important point (the zero) of the observations depends 

 not upon a thin spider line or engraved line, but upon the formation of 

 a bold black circular spot whose diameter is variable and dependent upon 

 the curvature of the lens employed. Where great endurance is desired, 

 a small plano-convex sapphire lens can be cemented to the prism at 

 little expense. 



The behaviour of minute microscopic kerosine oil-drops persistently 

 adhering to the surface of the lens, in spite of repeated wipings, is 

 worthy of notice. 



On the near approach of the lens to the under surface of the glass 

 under notice, the scattered drops suddenly coalesce, shooting out into a 

 film of varying colour. 



On one occasion a small oil- drop, one hundredth of an inch in 

 diameter, appeared as a black annulus enclosing a bright thin ring of 

 light, which enlarged on being touched by the prism-lens by the advance 

 of the screw. It spontaneously then spread out and rapidly exhibited 



