I06 rHILOSOPHICAL TRANSACTIONS. [aNNO I73S, 



for the different size of the horizontal moon at different times, adding also the 

 consideration of the faintness which vapours sometimes throw on the ap- 

 pearance. 



An Explication of the foregoing Experiment, to account for the Appearance 

 the horizontal Moon seeming larger than when higher. By the same. N° 444, 

 p. 392. 



Dr. Desaguliers having made an experiment with 3 ivory balls, for confirma- 

 tion of what he had advanced, namely, that the deception arises from our judg- 

 ing the horizontal moon to be much farther than it is ; which is as follows. 



Two equal ivory balls, fig. 11, pi. 4, were set one beyond another in respect 

 of the eye at e, namely, ab at 20 feet distance from the eye, and cd at 40. 

 Now it is certain, by the rules of optics, that the eye at e or p, will see the 

 ball CD, under an angle but half as large as it sees the ball ab ; that is, that the 

 ball CD must appear no larger than the ball op placed by the side of ab. But 

 when looking at the two balls with the naked eye in an open room, we consider 

 that CD is as far again from the eye as ab, and we judge it to be as large as ab, 

 as it really is, notwithstanding it subtends an angle but of half the size. Now 

 if, unknown to the spectator, or while he turns his back, the ball cd be taken 

 away, and another ball op of half the diameter be placed in the same line, but 

 as near again, at the side of ab, the spectator thinking this last ball to be at 

 the place of cd, must judge it to be as large as cd, because it subtends the 

 very same angle as cd did before. 



It follows therefore — that if a ball be imagined to be as far again as it really 

 is, we make such an allowance for that imagined distance, that we judge it to 

 be as large again as it is, notwithstanding the angle under which we see it, is 

 no greater, than when we look at it, knowing its real distance. For this reason, 

 the moon looks larger in the horizon, and near it, than at a considerable height, 

 or at the zenith : because it being a common prejudice to imagine that part of 

 the sky much nearer to us which is at the zenith, than that part towards the 

 horizon ; when we see the moon at the horizon, we suppose it much farther ; 

 therefore as it subtends the same angle nearly, as when at the zenith, we 

 imagine it so much larger as we suppose its distance greater. 



The reason why this experiment is difficult to make, is because the light from 

 the ball op is too strongly reflected on account of its nearness ; but if we could 

 give it so little light, as to look no brighter than the ball cd, it would deceive 

 every person. Dr. D. has made the experiment so as to deceive such as were 

 not very long-sighted ; but he found it very difficult to deceive those who see 

 at a great distance ; though they would all be deceived, if the distances were of 



