Immersion Lenses and Neto Refradometers. 



65 



V. — 0)1 the History, Befradions, Definition, and Powers of 

 Immersion Lenses and New Refradomeiers. By Eoyston- 

 PiGOTT, M.A., M.D. Cantab., M.E.C.P., FeUow of tlie Cam- 

 bridge Philosophical and the Koyal Astronomical and Micro- 

 scopical Societies of London, formerly Fellow of St. Peter's 

 College, Cambridge. 



{Fart I) 



I. — History has handed down to us the labours of Claudius 

 PTOLEiiT, the Father of Optics. His celebrated experiment of ren- 

 dering a coin visible placed in an empty vessel by Ming it with 

 water, was perhaps the first scientific attempt at experimental 

 refraction : thence he proceeded to measm-e the refi-action of hght 

 passing from water into air, from air into glass, and lastly from 

 water into glass. He sagaciously avoided the deviation from glass 

 into air by taking care the rays should emerge perpendicularly to 

 the surface of the glass. 



Bent Oar in Water. 



Refraction fioni Air into Water. 



His very ingenious mode of settling these points, as it reaUy 

 involves the important principle of the modern immersion lens, 

 may not be uninteresting to those unacquainted with the experi- 

 ments. Observing the crooked image of an oar in water, and the 

 curious apparent rising of the money as the water is poured into the 

 vessel, he had a semi-cylinder of " pure glass " constructed, to which 

 he adjusted a graduated circle, so that the diameter of the cyhnder 

 coincided precisely with the diameter of the graduated circle above 

 mentioned, and also with the surface of the water. A small 

 coloured body was placed at the centre of the circle; a second 

 similarly fitted to one of the quadrants out of the water ; a third 

 slided on the lower part of the graduated circle immersed in the 

 water. Observations were then made upon the angles of incidence 

 and refraction of such a degree of accuracy as to excite our admira- 

 tion after a lapse of nearly two thousand years. The determination 

 of the conditions of refraction through a water lens was thus 

 anciently efiected sufficiently exact for all the practical purposes of 

 modern theory. 



