VOL XLVIII.] PHILOSOPHICAL TRANSACTIOKS. 401 



inches, and the greatest part of that was the spray of the sea. And the mean 

 quantity for each month of the aforesaid 13 years, is as in the margin annexed. 



XLII. On the Fossil found at Dudley in Staffordshire, and described in the Phil. 

 Trans. N° 496. Bij Mr. Emanuel Mendez da Costa, F.R.S. p. 286. 



The famous fossil, which Dr. Lyttelton showed to the r. s. some time ago from 

 Dudley, and which is described in N" 496 of the Trans, caused many arguments 

 as to what class of animals it belonged-^ Dr. Pococke afterwards produced 2 or 3 

 specimens of it extended, which provdi it to be of the crustaceous tribe of ani- 

 mals. But none of his specimens being very perfect, M. da Costa here sends 

 a fair specimen of the said fossil extended, from the iron mines at Colnbrook-dale 

 in Shropshire, and which determines him to pronounce it to be the remains of a 

 crustaceous animal, of that kind called pediculi marini, which are scaled all 

 round, and can at will roll themselves up : and this particular kind may be justly 

 denominated pediculus marinus major trilobos. See fig. 8, pi. Q. 



Though he before thought it not described by any English author, yet he finds 

 it described and figured, though badly, by Mr. Edw. Lhuyd, in his Lithophy- 

 lacium Britannicum Ichnographicum, Epist. 1, p. 96, table 22 ; who found them 

 in plenty in quarries, juxta aedes nob. v. D. Gryfidii Rice de Newton, arm. prope 

 oppidum Sancti Teilavii, in comitatu Maridunias. He calls it buglossa curta 

 strigosa. He also gives the figure of it without any description, in the Phil. 

 Trans. N° 243. 



XLJIl. Letters relating to a Theorem of Mr. Euler, of the Royal yfcad. of Sci- 

 ences at Berlin, and F. R. S. for Correcting the Aberrations in the Object- 

 Glasses of Refracting Telescopes, p. 287. 



Letter L From Mr. James Short, F.R.S. to Peter Daval, Esq. F.R.S. Dated 



April Q, 1752. p. 287. 



There is published, in the Memoirs of the Royal Acad, at Berlin, for the year 

 1747, a theorem by Mr. Euler, in which he shows a method of making object- 

 glasses of telescopes, in such a manner, as not to be aflTected by the aberrations 

 arising from the different refrangibility of the rays of light ; these object-glasses 

 consisting of tv/o meniscus lenses, with water between them. 



Mr. John Dollond, who is an excellent analyst and optician, has examined 

 the said theorem, and has discovered a mistake in it, which arises by assuming 

 an hypothesis contrary to the established principles of optics ; and in consequence 

 of this Mr. Dollond has sent me the inclosed letter, which contains the disco- 

 very of the said mistake, and a demonstration of it. 



In order to act in the most candid manner with Mr. Euler, I have proposed 



VOL. X. 3 F 



