OPTIME 



When light pane* from one 

 transparent medium to another, as 

 from air to glass or water, or vice 

 vena, it is found not only that 

 we have the phenixni-ixm of ns . 

 flection, but alto that the portion 

 >f tli,- li.jit which enters the second 

 iiu-iliiiin is in general diverted from 

 This occurrence 



is called refraction, and in the dis- 

 cussion of it the concept of a ray 

 enables us to reduce our knowledge 

 concerning refracted light to two 

 simple geometrical statements : 

 (1) the incident ray, the refracted 

 ray, and the normal are in one 

 plane ; (2) the sine of the angle of 

 incidence bears to the sine of the 

 angle of refraction a ratio which is 

 definite for two given media. Thus 

 from air to glass this " index of 

 refraction " is 3/2 ; from glass to 

 air 2/3 ; air to water 4/3 ; water to 

 air 3/4 ; water to glass 9/8 ; glass 

 to water 8/9 ; and so on. Each 

 pair of substances has its definite 

 ratio. The images produced by the 

 lenses in reading-glasses, telescopes, 

 cameras, etc., are due to the 

 deviations of the rays of light 

 emitted by a source, and their re- 

 direction "into new paths which 

 pass " really " or " virtually " 

 through the points of the images 

 which are actually seen by us ; and 

 experiment confirms the fact that 

 all such appearances can be ar- 

 rived at deductively by the 

 methods of geometry, if we apply 

 the two simple statements enun- 

 ciated above to the given con- 

 ditions in any case. 



As stated, geometrical optics 

 gives a very adequate theoretical 

 explanation of the facts concern- 

 ing image formation, but it is 

 quite inadequate to explain a large 

 number of observed results of 

 experiment in optical science. The 

 phenomena of interference, dif- 

 fraction, polarisation, and even 

 chromatic dispersion are quite 

 beyond its scope. As a simple state- 

 ment of intenereVice we can say 

 that it is possible for two specially 

 adjusted sources of light to produce 

 darkness along certain paths, this 

 being compensated by reinforced 

 illumination along others. In 

 diffraction we deal with the exist- 

 ence of certain colour fringes well 

 inside the geometrical shadow 

 cast by the edge of an opaque ob- 

 ject. In polarisation we meet with 

 the phenomenon of double re- 

 fraction by certain crystals (e.g. 

 Iceland spar, fluor spar, mica, 

 aragonite), each of the two re- 

 fracted rays arising from one in- 

 cident ray being plane-polarised; 

 meaning that for each refracted 

 ray there is a particular plane 

 containing the ray which has im- 

 portant physical properties in any 



3837 



OPTOPHONE 



subsequent treatment of the ray. in his Thendide (justification of 



! the ray is directed on to a God), endeavours to prove that 



piece of glass at a definite angle. God, in His infinite intelligence, 



.in. I if -I,.- " plane of polarisation " conceived an infinity of possible 

 of the ray is at right angles to the 



worlds, and called into being the 

 one which He regarded, all things 

 considered, as the best But even 

 this does not exclude the idea of in- 

 finite perfectibility. See Pessimism. 

 Option (Lat, optare, to choose). 

 Literally, the act of choosing. In 

 financial language an option is the 

 right to buy or sell something, 



, for which right money is paid. 



hypothesis as to the undulating Thus a man pays for the right to 



plane of incidence on the glass, no 

 reflection takes place, the entire 

 energy of the ray being trans- 

 mitted into the glass. 



In such phenomena, no adequate 

 explanation is possible without the 

 help of a definite theory as to the 

 nature of light. The treatment of 

 such experimental data and the 



nature of light which has developed buy certain shares at a certain 

 during the past century and a half price, or to buy a house or property 

 are treated in works on physical o f other kinds. If he does not wish 



optics. In them appeal has to be 

 made to mathematical analysis of 

 a type much more complex than 

 that involved in the comparatively 

 simple geometry employed in 

 geometrical optics. 



Bibliography. Lessons in Light 

 and Heat, D. E. Jones, 1892 ; Heat 

 and Light, R. T. Glazebrook, 1894 ; 

 Theory of Light, T. Preston, new 

 ed. 1901 ; Light for Students, E. 

 Edser, 1902 ; Physical 'Optics, 

 R. W. Wood, 1911; Treatise on 

 Light, R. A Houston, 1915 ; General 

 and Practical Optics, L. Lawrence, 

 3rd. ed., 1920. 



Op time (Lat, excellently). Ad- 

 verb taken from the phrase optime 

 mcruit, he has deserved very well, 

 and used at the university of Cam- 

 bridge as a name for candidates for 

 honours who are placed in the 

 second and third classes of the 

 mathematical tripos. They are 

 known as senior and junior op times 

 respectively. Pron. Optim-ee. 



Optimism (Lat, opt imus, best). 

 The tendency to regard things in 

 the most favourable light, opposed 

 to pessimism. In philosophy, the 

 doctrine that this world is the best 

 of all possible worlds. There are 

 two kinds of optimism, relative and 

 absolute. The world may not be 

 absolutely good, but at least the 

 good in it is predominant Leibniz, 



Optophone. Blind person reading by means ol the instru- 

 ment which converts optical into phonetic eflects 



to exercise his option he loses the 

 money paid down. An option to 

 buy is termed a call, and an option 

 to sell a put. See Stock Exchange. 

 Optophone. Instrument, in- 

 vented by E. E. Fournier D'Albe, 

 which converts optical into phonic 

 or sound effects, to enable blind 

 persons to read printed matter by 

 ear. It is based upon the chemical 

 element selenium, which possesses 

 the property of offering to electri- 

 city passing through it a resistance 

 which varies with the degree of 

 illumination to which it is subject 

 The instrument consists of a 

 brass disk perforated with eight 

 rings of holes, the number of holes 

 in the rings being proportional to 

 the vibrations in the notes of a 

 musical octave. The disk revolves 

 adjacent to an inclined cover con- 

 taining a row of eight holes which 

 register at intervals with those in 

 the disk, and over the perforations 

 in the cover is passed a strip of 

 gelatin upon which is printed in 

 large block type the matter to be 

 read. Light from a strip-light 

 electric lamp is concentrated by a 

 water tube and passed through per- 

 forations in the revolving disk and 

 those perforations in the row in the 

 cover not obstructed by the 

 type. The light .traversing the 

 perforations is re- 

 ceived upon a 

 selenium tablet 

 connected with a 

 telephone receiver 

 through a high- 

 voltage battery. 

 By this means each 

 letter as it passes 

 the row of perfora- 

 tions is accom- 

 panied Jby a 

 c bar acteristic 

 sound in the re- 

 ceiver, which 

 sounds can be 

 recognized after a 

 certain amount of 

 practice, and the 

 printed matter thus 

 interpreted. 



1H 1 



