1876.] a new Refractometer. 395 



venience was accorded by using a minute film of paraffin-oil, formed by 

 repeatedly wiping the surface of the screw-lens. Contact was then 

 heralded instantly between the two surfaces by a brilliant flash of 

 colours. One thing appeared certain, the various colours could be pro- 

 duced in perpetually expanding and vanishing rings, always starting 

 from the centre. I counted no less than thirty-two changes of colour 

 in the central part, reckoning from the black spot of perfect contact 

 (within the half-millionth of an inch) and the final evanescence of the 

 last colour by slight movements of the screw. 



Eor the purpose of illuminating the point of contact of the " screw- 

 lens " with the plate under examination, I inserted a minute right-angled 

 prism behind the lens. This giving totally reflected light, provided 

 nothing adhered to the reflecting surface of the prism, afforded the 

 means of making observations by ordinary daylight, and observing the 

 rings of Newton, though very pale, by transmitted light. 



Another great advantage of the prism-lens attached to the end of the 

 steel measuring-screw, is the bright illumination of the contact surfaces, 

 the contact of an opaque extremity of the screw being with difficulty 

 ascertained. 



The whole method of finding the refractive index of a thin plate of a 

 given refractive material resolved itself, then, into the best instrumental 

 means for advancing a fiducial visible and illuminated point truly and 

 steadily through measurable intervals, and observing with a good micro- 

 scope the precise position of evanishment in and out of focus, and deter- 

 mining the focal points under correct collimations. 



After many trials the following form was adopted : — 



A steel cylinder very accurately turned between dead centres (i. e. the 

 centres being fixed and the object revolving between them), about 5 

 inches long and -f§ in diameter ; upon this screw-threads (very nearly 

 101'3 per inch) were very patiently formed. The front part of the cylinder 

 passes smoothly (at first air-tight like a piston) through a collar of brass, 

 into which it had been very slowly and carefully ground (with the finest 

 cutting-powder and oil) ; at about two inches of the other end was formed 

 a screw as described, with a very exact apparatus lent to the writer by 

 a celebrated optician. 



It was found that in so delicate an operation as dealing with coloured 

 films, touching any part of the instrument caused them to flash a new 

 colour. It was necessary to obviate, then, all varying mechanical strain. 

 Springs so common in micrometers to obviate " loss of time" were found 

 to introduce, from their varying pressure, very variable errors : after a 

 time I was compelled to abandon their use altogether. 



The constant force of gravitation and dead unvarying weight was now 

 introduced ; and in order to compensate possible deviations in the true 

 spiral form of individual threads (every one of which was carefully exa- 

 mined with a strong rnagnifying-power), a nut was formed, so as con- 



2g2 



