102 



SCIENCE, 



[Vol. IV., No. 78. 



The important point is, whether equally good defini- 

 tion can be attained with these two extra reflections. 

 Experience alone can decide this with certainty; but 

 up to the limit already tried in the Paris instrument, 

 about ten inches aperture, we have the strongest 

 evidence of its possibility. Dr. Gill, astronomer at 

 the Cape of Good Hope, in describing to the Royal 

 astronomical society a flying visit to continental 

 observatories, speaks of the Paris equatorial coude 

 as follows: — 



" One fine night, about eleven o'clock, we went to the obser- 

 vatory, and set on y Leonis; and I am bound to say I never saw 

 the diffraction-disks of a star better defined than in that instru- 



Also it would seem, that if large lenses, whose 

 thickness is limited, can be supported at the rim so 

 that the distortions due to gravity are not appreci- 

 able in the definition, then mirrors whose thickness 

 is unlimited, and which can be supported in every 

 possible way at the rim, and all over the back surface, 

 might be made sufficiently rigid to resist distortion. 

 To be sure, the effects of distortion are of quire a 

 different order in the two cases, the effect of gravity 

 in increasing the curvature of one side of a lens 

 being partly counteracted by the diminished curva- 

 ture of the other side, while the distortion of a 

 single reflecting surface appears with its full effect 



equatorial coude at the Paris oeservatort. (Reproduced from VAstronomie.) 



ment. They were perfectly circular. The disks came as sharply 

 to focus as any I ever saw; and I would not have believed, if I 

 had not seen it, that it was possible to make an instrument in 

 which, after two reflections, such definition could be found. I am 

 bound to say I never saw better definition in any instrument, and 

 I never measured a double star so pleasantly and easily before." 



Dr. Gill's well-known investigations in stellar as- 

 tronomy give to whatever he says in this line great 

 weight, and no stronger testimony could be desired. 



When it comes to the question of the largest aper- 

 tures, it would seem, a priori, that there should be 

 no difficulty in making a glass mirror — where the 

 internal constitution of the glass is not in question, 

 and only one plane surface is demanded — 1.41 times 

 as large as an objective, in which the glass of the 

 two lenses must be homogeneous throughout, and 

 four perfect surfaces are required. 



in the definition. But the far greater facilities for 

 making the mirrors rigid should make up for this 

 in a large degree. At any rate, the French opticians 

 seem to have full confidence in their ability to do this, 

 and it is certainly to be hoped that they will succeed. 



Washington. H. M. PAUL. 



THE ECHINODERMS DREDGED BY THE 

 TALISMAN. 1 



Among the deep-sea echinoderms, some of the 

 holothurians attain a large size, one being seventy 

 centimetres long. The mouth is situated at one end 

 of the body, although near the termination of the 



1 Abridged from the French of H. Filhol in La Nature. 



