250 



SCIENCE, 



[N. S. Vol. XXIV. No. 608. 



sculpturing. The microscopic structure has 

 not yet been examined. 



In general the form, size and arrangement 

 of the ossicles are much as in the bones in the 

 Grypotherium skin from Patagonia. The 

 skin fragment first described by Woodward 

 was thought to represent mainly the region 

 of the neck and shoulder. The Californian 

 specimen mantles over the outer side of the 

 scapula, and is presumably not far removed 

 from its original position with relation to this 

 bone. The generic position of the form rep- 

 resented by this specimen appears as yet some- 

 what uncertain, but a satisfactory determina- 

 tion of its affinities will probably be possible 

 when the skeletal material available has been 

 finally assembled. John C. Merbiam. 



A SUGOESTION FOR INTENSIFYING THE DOPPLER 

 EFFECT. 



It has never been pointed out, I believe, 

 that the relation between the conjugate foci 

 of a concave mirror furnishes, at least in 

 theory, a means of enormously intensifying 

 the Doppler effect. If we have a source of 

 light a little outside the principal focus of a 

 concave mirror we shall have an inverted 

 image formed at a considerable distance; and 

 if the source move toward or from the mirror 

 the image will move in the opposite direction 

 with a much greater speed. ' 



Let fi and f, be any two conjugate focal dis- 

 tances, and F the principal focal distance; 

 then 



Differentiate with respect to t; 

 I df, I df 



A' dt /,2 dt 



= 



Writing v^ and v^ for the speeds of the 

 source and the image, we have 



^± = -11 



that is, the speeds are proportional to the 

 squares of the distances from the mirror. 



To show the theoretical possibilities of this 

 formula let us suppose a source of light 



moving with a speed of 10' cm. per second at 

 a distance of 10 cm. from the mirror, whose 

 focal length, of course, must be a trifle less 

 than this figure; at what distance must the 

 image be formed in order that its speed shall 

 be 10'°, one third of the speed of light? 



102 



10'° 



103 



a; = 316 meters, nearly, a distance obtainable 

 in the laboratory with a moderate number of 

 reflections. 



The chief difficulty to be overcome in any 

 experiment of this nature would be the faint- 

 ness of the image due to its great size. A 

 continuously moving source of light could 

 be obtained either by a wheel with mirror 

 teeth or with a self-luminous rim. 



Paul R. Heyl. 



Centeal High School, Philadelphia. 



TEE COMPULSORY RETIREMENT OF THE 



DIRECTOR OF THE BRITISH MUSEUM 



OF NATURAL HISTORY.' 



In a letter which we publish to-day Pro- 

 fessor Hay Lankester, who is this year presi- 

 dent of the British Association, tells the story 

 of the summary termination of his director- 

 ship of the Natural History Museum, some 

 imperfect versions of which have obtained 

 currency. The standing committee of the 

 trustees have taken advantage of the civil 

 service rule that the head of a department 

 may call upon any officer in it to retire at the 

 age of sixty, upon such pension as he is en- 

 titled to by the general regulations. That 

 rule is not usually acted upon in the absence 

 of some special reason, unless the officer has 

 completed such a term of service as entitles 

 him to the maximwm pension. Dr. Lankester 

 was appointed at the age of fifty-two, so that 

 when he reaches sixty next May a regulation 

 intended to apply to men who have spent their 

 lives in a government office decrees that his 

 pension shall be £160, which the treasury of 

 its goodness may raise to £300. In any coun- 

 try but this it would be thought grotesque 

 and monstrous that a distinguished man of 

 science asked to serve the state after the age 



^ From the London Times. 



