s«ct. v.] DISSERTATION SECOND. 173 



which they are supposed to be, on account of the number 

 of intervening objects across which they are seen. Ptole- 

 my's explanation, however, as stated by Delambre, 1 from the 

 manuscript just mentioned, is quite different from this, and 

 amounts to no more than the vague and unsatisfactory re- 

 mark, that an observer looks at the bodies near the zenith 

 in a constrained posture, and in a situation to which the 

 eye is not accustomed. The former explanation, therefore, 

 given by Alhazen, but supposed to have been borrowed 

 from Ptolemy, must now be returned to its right owner. 

 It is the best explanation yet known. 



These are the only mathematical treatises on opticks of 

 any consideration which the ancients have transmitted to 

 us; 2 but many metaphysical speculations on light and 

 vision are to be found in the writings of the philosophers. 

 Aristotle defined light much as he had defined motion ; the 

 act or energy of a transparent body, in as much as it is 



1 Connaissance des Terns, 1816, p. 245. &c. The glimpses 

 of truth, uot destined to he fuily discovered till many ages after- 

 wards, which are found in the writiugs of the ancients, are always 

 interesting. Ptolemy distinguishes what has since heen called 

 the virtual focus, which takes place in certain cases of reflection 

 from spherical specula. He remarks, that colours are confounded 

 by the rapidity of motion, and gi^es the instance of a wheel 

 painted with different colours, and turned quickly round. 



2 Another Greek treatise on opticks, that of Heliodorus of 

 Larissa, hns been preserved, and was first published by Erasmus 

 Bartholinus at Paris, in 1657. It is a superficial work, which, 

 to a good deal of obscure and unsound metaphysicks, ad«is the 

 demonstration of a few very obvious truths. The author holds 

 the opinion, that vision is performed by the emission of some- 

 thing from the eyes ; and the reason which he assigns is, that 

 the eyes are convex, and more adapted to emit than to receive. 

 His metaphysicks may be judged of from this specimen. He 

 ha3 not been made mention of by any ancient author, aud the 

 time when he wrote is unknown. As he quotes, however, the 

 writings of Ptolemy and Hero, he must have been later than the 

 first century. 



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