OPTIC NERVE 



5856 



OPTICS 



a diploma and the right to append 

 the initials F.S.M.C. The British 

 Optical Association grants a certi- 

 ficate and the right to append 

 D.B.O.A. or F.B.O.A. The Fellow- 

 ship of the Institute of Oph- 

 thalmic Opticians is open only to 

 sight testing opticians who are 

 qualified by either of the bodies 

 mentioned. In the U.S.A., and in 

 many of the provinces of Canada 

 and Australia, compulsory quali- 

 fication and registration are re- 

 quired before a person can engage 

 in the practice of sight testing. 

 See Eye.; Sight. 



Optic Nerve. Nerve of sight. 

 Arising from the lower part of the 

 brain, it passes forwards into the 

 orbit, where it enters the eyeball, 

 and its fibres spread out over the 

 inner surface of the retina. In- 

 flammation of the optic nerve, or 



optic neuritis, is a serious affection 

 which most frequently arises in the 

 course of tumours or other affec- 

 tions of the brain, or Bright's 

 disease. Optic atrophy is a de- 

 generation of the nerve fibres 



rOPTIC NERVE 



Optic Nerve. Diagram showing 

 position of optic nerve from above 



which may follow optic neuritis, or 

 may be due to tabes dorsalis (loco- 

 motor ataxia) and other nerve dis- 

 eases. The condition eventually re- 

 sults in blindness. See Eye ; Nerve. 



OPTICS: SCIENCE OF LIGHT AND VISION 



James Bice, M.A., Lecturer in Physios, Liverpool University 



Among the articles which supplement the information given below 

 are Dispersion ; Lens ; Light ; Refraction ; Relativity ; Spectrum 



The most familiar phenomena 

 of vision are o'ur inability to see 

 objects round corners ; the forms 

 of shadows cast by opaque bodies ; 

 the formation of images of 

 luminous objects by mirrors and 

 the lenses of such instruments as 

 reading glasses, telescopes, micro- 

 scopes, cameras and projecting 

 lanterns. A great deal of the 

 knowledge we possess concerning 

 these matters is very satisfactorily 

 summarised in a few principles 

 which are referred to as the laws 

 of Geometrical Optics ; these are 

 statements which if assumed to be 

 true will lead by the deductive 

 methods of pure geometry to con- 

 clusions which are in very exact 

 accord with experimental results. 

 In this branch of optical science no 

 hypothesis is made as to the 

 nature of light beyond the simple 

 assumption that it is an influence 

 emitted from all visible bodies and 

 capable of affecting the retinae of 

 our eyes. 



Umbra and Penumbra 



Very little observation is re- 

 quired to convince us that this in- 

 fluence is propagated in straight 

 lines. If one examines the shadow 

 cast by an opaque body on a white 

 screen in a room lit by one source 

 of light only, an intensely black 

 central part, called the umbra, 

 is, in general, observed, around 

 which is spread a less dark part, 

 gradually fading into the complete 

 illumination of the rest of the 

 screen ; this annular, partially illu- 

 minated portion of the shadow is 

 called the penumbra. 



Careful investigation will show 

 that it is impossible to draw a 

 straight line from any point on the 



surface of the luminous source to 

 any point in the,umbra which does 

 not cut through the opaque object, 

 and that if we select any point in 

 the penumbra it is possible to 

 draw straight lines to points on a 

 portion of the luminous source 

 which do not meet the opaque 

 object, the portion of the source 

 becoming larger as the point chosen 

 in the penumbra moves further 

 out from the umbra, until we 

 reach those parts of the screen in 

 full view of the complete source. 

 In particular, if the source is of 

 very small dimensions, the pen- 

 umbra is so small as to be imper- 

 ceptible except at very close 

 quarters, and the shadow consists 

 almost entirely of an umbra. 



Eclipses as Examples 

 The eclipses of the sun and 

 moon are examples of shadows on 

 an enormous scale, e.g. during 

 total eclipses of the sun there is at 

 any instant one comparatively 

 small portion of the earth where 

 the eclipse is actually total ; this 

 portion is the umbra of the shadow 

 cast by the moon. Outside this 

 lies a ring shaped portion, the 

 penumbra, where the eclipse is only 

 partial. Owing to the rotation of 

 the earth, these portions move 

 over the earth's surface, and thus 

 we see the reason for the exist- 

 ence of the " track of totality." 

 As a further illustration of rectili- 

 near propagation we may instance 

 the familiar " streaks " in front of 

 a projecting lantern or within a 

 fairly dark room into which sun- 

 light is streaming through a 

 window ; but in this connexion it 

 may be well to remove a common 

 misconception. One is not " seeing 



light " in these circumstances ; 

 such a phrase betokens a con- 

 fusion of ideas ; what we see is the 

 dust and motes in the atmosphere 

 which are being more than usually 

 illuminated, and the cylindrical or 

 conical form of these streaks arises 

 from the rectilinear paths pursued 

 by the elements of light emitted 

 by the source. The fact of rectili- 

 near propagation leads naturally 

 to the use of the phrase, a beam of 

 light, and still further to the con- 

 ception that such beams are com- 

 posed of extremely narrow beams 

 which we idealize as straight lines 

 and call rays. 



Principles of Reflection 



This concept of a ray is in- 

 valuable in the study of reflection 

 and refraction of light. As a rule 

 reflection from the surface of a 

 body is quite irregular ; the rays 

 trom a self-luminous source when 

 they reach walls, floor, ground, etc. 

 are scattered and redirected in all 

 directions, otherwise such non- 

 luminous, but illuminated, objects 

 would not be visible from all 

 points of view, as they actually are 

 unless opaque bodies intervene. 

 But provided the surface of a 

 body has a certain amount of 

 polish or smoothness, we begin to 

 observe traces of regular reflection. 

 The appearance of a well-polished 

 table is an example of such partial 

 regular reflections ; and when the 

 polish reaches that of the best glass 

 or a brilliant metallic surface, 

 practically all the reflected light is 

 regularly reflected, and wo get the 

 phenomena of images. 



In such cases each reflected ray 

 makes the same angle as its in- 

 cident part with the " normal," 

 i.e. the line perpendicular to the 

 reflecting surface at the point of 

 incidence, and the incident ray, 

 reflected ray and normal lie in 

 one plane. Such a deviation from 

 their original paths causes a small 

 cone of rays, which could enter the 

 pupil of an eye, originally emitted 

 by a minute portion of a luminous 

 or illuminated body (a " point- 

 source "), to appear on its re- 

 ception by an eye to be diverging 

 from quite another point. The 

 aggregate of such " point-images " 

 forms the image of the body which 

 is the aggregate of the original 

 " point-sources." In certain cases 

 of curved mirrors, a cone of rays 

 originally diverging from a point- 

 source may be made to converge by 

 reflection and thus pass through 

 a point onl} 7 to diverge once more 

 from what is called a "real" 

 image which is actually in front of 

 the mirror and not behind it. The 

 reader may verify this for himself 

 by looking into the hollow of a 

 spoon with a lamp near at hand. 



