20 



NATURE 



\_Nov. 7, 1878 



its being situate in the section of maximum luminosity of the 

 flame, is a somewhat delicate one, and ought, if possible, not to 

 be disturbed oftener than is necessary for changing the lamp. 

 Moreover, while the raising of the lamp would depress the light 

 which passes through the refracting portion of the apparatus it 

 would have precisely the opposite effect upon the portions which 

 pass through the totally reflecting prisms placed above and 

 below the refracting part, which would then throw the rays 

 upwards to the sky, where they would be useless. But any 

 desired change could be effected by surrounding the flame with 

 prisms spheric on their inner faces, and concentric with the foci 

 of the different parts of the apparatus, so as to depress the rays 

 before they fall upon the main apparatus. Those prisms which 

 subtend the lens would have their thicker ends lowest, and those 

 subtending the reflectors would have their thicker ends upper- 

 most. 



The great disadvantage, unless in the case of electric lights, 

 of employing the temporary apparatus which has just been 

 described, arises from the loss of light by divergence, due to the 

 relation subsisting between the radius of the flame and the radii 

 of curvature of the apparatus itself. 



But this loss may be prevented by another plan. Outside of 

 the apparatus, and either close to it, or what would be more 

 convenient, close to the glazing of the lantern, movable re- 

 fractors made of panes of plate glass could be placed during 

 fogs. In ordinary states of the weather, these fog screens, 

 which would be hung by chains passing over pulleys fixed to the 

 top of the lantern, would be close to the inside of the parapet 

 wall of the light-room, and below the apparatus. If these 

 refi-actors were fitted with counter-weights they could, in the 

 course of a few minutes, be hauled up in front of the apparatus 

 by the hand when the thick Aveather came on, and pulled down 

 again when it became clear. The panes of plate glass which act 

 as refractors, would be of prismoidal section vertically having 

 their thicker ends placed downwards. The vertical angle of the 

 prisms would in each case depend on the height of the light- 

 room above the sea, and the distance off shore to which the 

 strongest beam of light required to be dipped during fog. But 

 after more fully considering the question, I have come to the con- 

 clusion that a great improvement could be effected even without 

 resorting to temporary expedients. From a series of observa- 

 tions made with two kinds of photometer by Messrs. Stevenson, 

 in 1865, on the penetrative power of light from a first order lens 

 and cylindric refractor, it appears that for an angle of 0° 30' in 

 altitude above the plane of maximum intensity, and for 0° 30' 

 below that plane, the power of the light does not vary more 

 than at greatest from 4 to 6 per cent., and that if the strongest 

 part be sent to the horizon, about one -half of the whole is sent 

 uselessly to the skies. 



Powsr of Lens in the Vertical Plane. 



Means of four sets 

 , ' of observations. 



o 40 above the level of maximum '90 



o 30 » » M *94 



020 „ „ „ '97 



o 10 „ „ „ -98 



o o maximum power i "00 



o 10 below the level of maximum "99 

 o 20 „ „ „ -97 



030 „ ., „ '96 



Note. — These results, which are the means of four sets of 



observations, did not extend further in the vertical plane. 



Result of Dipping Light as Proposed, Contrasted with Present 

 System. 

 Present System. Proposed System. 



ABove horiion. Power. Above horizon. Power. 



Applying these observations, so far as they extend in the 



vertical plane, to the case of lighthouses elevated much above 

 sea-level, we see that to dip the strongest beam to a point much 

 nearer the shore than the sea horizon, while it would not appre- 

 ciably affect the visibility there, would even, so far as the 

 observations go, increase the power of the light nearer the shore. 

 Those who have been close to a lighthouse on a hazy night must 

 have noticed the luminous rays passing through the air far above 

 the sea-level, and cases are adduced by Mr. Beazeley of ship- 

 wrecks having occurred when the light could not be seen by the 

 sailors, although their vessels were stranded close to the tower. 

 As the lens has the greatest divergence, and is the only agent 

 for giving light near the shore, it only should be dipped 

 so as to throw as few of the rays as possible uselessly on 

 the skies, while the reflecting prisms, which have much less 

 divergence, will remain as at present throwing their rays , 

 to the horizon. By this different distribution of the light 

 from the lens and the prisms, although the strongest beams from 

 the lens were dipped 0° 20' below the horizon, which cau~es a 

 loss there of 3 per cent, of lens power, yet the loss on the 

 -whole light coming from both lens and prisms, taken at Mr. 

 Chance's valuation of 70 and 30 respectively, will be reduced to 

 only about 2 per cent., while the sea near the shore will be more 

 powerfully illuminated than at present. It may, however, be 

 fairly questioned whether the strongest beam ought not to be 

 dipped to 0° 30', as this would still further increase the power 

 near the shore, and would only depreciate the light at the hori- 

 zon by about 5*8 per cent. It is well to remember that, should 

 the flame, through neglect of the keeper, fall at any time below 

 the standard height, such a defect will operate most injiu-iously 

 on the light falling near the shore, and not so much on that sent 

 to the horizon. Now there can be no question that in all ordinary 

 cases a vessel with such an ofiing as twenty miles, which is the 

 sea-range due to 300 feet of elevation, is in a far safer position 

 than if she were within a mile or two of the shore, and hence 

 the propriety of increasing the light near the shore so long as 

 we do not to any appreciable extent reduce it at the horizon. 1 



T. Stevenson 



MEDICAL ENDOWMENTS AT OXFORD 



WE have been requested to publish the following 

 details of existing endowments assigned by founders 

 to the study of Medicine and of Human Anatomy and 

 Physiology as bearing on Medicine : — 



I. The Regius Professorship of Medicine, as at present con- 

 stituted, is worth about 500/. a year. The items are : (i) from 

 the Queen's exchequer, 35/. ; {2) as Master of Ewehne Alms- 

 house, 250/. ; (3) as Aldrichian Professor of Medicine, 126/. ; i 

 (4) examination and graduation fees, 70/. to lOo/. 



II. Lord Lichfield's Clinical Professorship, which is not united 

 with the Regius Professorship, is worth 200/. a year. 



Dr. H. W. Acland holds both the Regius and the Cliracal^j 

 Professorships : no instruction is given by Dr. Acland in either 

 capacity. 



III. The Linacre Professorship ofPhysiology and Anatomy has-j 

 absorbed the old foundations for the encouragement of humar 

 anatomy, namely, the Tomlinsian Prrelectorship and thejj 

 Aldrichian Professorship. It is worth 800/. a year, the sural 

 which Merton College pays in place of the original endowments 

 entrusted to it by Thomas Linacre, founder of the College of| 

 Physicians, and once a lecturer on medicine in Oxford. The 

 Linacre Professor is engaged in teaching Comparative Anatomj 

 to candidates for the B.A. degree. 



IV. A separate Demonstratorship of Anatomy, worth 200/ii 

 a year, also still exists, and was intended by the commissioners 

 of 1852 to provide for the teaching of human anatomy, asl 

 designed by Tomlins and Aldrich. The gentleman who holds| 

 this post is Curator of the Museum of Comparative AnatomyJ 

 and does not teach Human Anatomy. | 



V. The beautiful old Physic Garden founded by Earl Danbyl 

 in 1622 is another heirloom of the Medical Faculty of Oxford.-ii 

 The chair of Botany was endowed by Dr. Sherard and the Collegej 

 of Physicians of London elect the professor. By special pro- - 

 vision, the clergy are excluded from this professorship, and 

 preference is to be given to a tnedical graduate. The chair is now 

 worth, with later additions, about 400/. a year. 



VI. Lastly, a very important trust fund is administered by the 

 governing body of Christ Church, the bulk of which was left by 

 Dr. Matthew Lee in 1755 to provide for anatomical teaching in. 

 relation to medicine exclusively. Dr. Lee's expression of his 



