J 34 PHILOSOPHICAL TRANSACTIONS. [ANNO 1781. 



of the telescope, and of the rays passing between the reflectors, being parallel to 

 that plane; whereas the inclination of the telescope, and of the intermediate 

 rays, as well as of the reflectors themselves to the plane of motion, admit of 

 unlimited variety. A general theory to determine the angle observed by 1 re- 

 flections, from the data on which its magnitude depends, without limitation or 

 restriction, seems applicable to several useful purposes in practical astronomy. 

 Having never seen any geometrical construction or analysis of this curious pro- 

 blem, Mr. A. was induced to bestow some consideration on the subject. And it 

 must be acknowledged that his labours have produced a long and elaborate 

 essay, more fit for a separate volume, than for a paper in the Philos. Trans, or 

 these Abridgments; and of which the intricate constructions and analytical 

 calculations could be of little or no use to the mere practical optician and 

 astronomer. 



XXF1II. On the Ophidium Barbatum Linnei. By P. M. Augustus Broussonet, 



M. D. p. 436. 



This species of fish seems not to have been unknown to the ancients, though 

 probably they confounded it with the Conger, to which it bears some resem- 

 blance. Perhaps the early Greek and Latin writers on natural history have men- 

 tioned it under the name of Tragus, or Callarias ; but, for want of descriptions, 

 they left us much in the dark concerning it. Pliny indeed speaks of a fish 

 which appears to be of this species : he calls it Ophidion, and as that is the 

 name given to it by all the modern writers, we are obliged to accept his synonymy 

 without further inquiry. 



The first author to whom we are indebted for a description and figure of the 

 ophidium, is Bcllonius ; yet it appears, that he was not certain of the name of 

 this fish, since he calls it gryllus, falso congrus, tragus, aselli species: nor was 

 he less doubtful of the class to which he should refer it, and therefore placed it 

 among the aselli, or gadi, though very different from the species of that family. 

 Rondeletius, who wrote soon after Bellonius, has given a better description, and 

 a more accurate figure of this fish, which he calls ophidion, with a reference to 

 Pliny. In the figure of Bellonius the cirri are very ill represented, and the 

 whole fish appears without any spots, whereas in the plate of Rondeletius it is 

 covered with oblong spots. This remarkable difference between the figures of 

 these authors was sufficient to determine Gesnerus, and others who have written 

 since their time, and who are to be considered rather as compilers than authors, 

 to take the fish described by Bellonius to be a different species from that of 

 Rondeletius. 



Willoughby, who is the first ichthyologist who has given any good descrip- 

 tion of fish, treats largely of the ophidium ; and in his account describes the 



