VOL. XVI.] PHILOSOPHICAL TRANSACTIONS. 329 



And it is undoubtedly from the same principle, that to the south of the 

 equator, in a part of the Indian ocean, the north-west winds succeed the 

 south-east, when the sun draws near the tropic of Capricorn. But I must con- 

 fess, that in this latter occurs a difficulty, not well to be accounted for, which-, 

 is, why this change of the monsoons should be any more in this ocean, than in' 

 the same latitudes in the Ethiopic, where there is nothing more certain than a 

 south-east wind all the year. 



It is likewise very hard to conceive why the limits of the trade wind should 

 be fixed about the 30° of latitude all round the globe ; and that they should so 

 seldom exceed or fall short of those bounds ; as also that in the Indian sea, only) 

 the northern part should be subject to the changeable monsoons, and that iii 

 the southern there should be a constant south-east wind. 



A Dioptric Problem, why four Convex-glasses in a Telescope show Objects erect. 

 By William Molyneuv, of Dublin, Esq. R. S. Soc. N° 183, p. 169. 



In the Journal des Sqavans for Sept. 17, l685, we find this passage: " As 

 perspectives of one convex glass make objects appear upright, which those of 

 two convex-glasses invert, and again those of three rectify ; so it should seem 

 that those of four ought to invert: and yet experience shows us that objects 

 appear upright through these glasses. The singularity of this phenomenon 

 obliges all skilled in dioptrics to inquire the reason of it, but hitherto they have 

 found none. Mr. Regis, who applies himself particularly to this part of Natu- 

 ral Philosophy, believes that he has hit upon the reason, and makes us hope 

 that he will suddenly publish it." 



To me this phenomenon appears very easily explicable, from the considera- 

 tion of placing glasses in a tube ; which is thus: after the object-glass, the eye- 

 glass is placed so much distant, towards the eye, from the focus of the object- 

 glass, as is the, focus of the eye-glass ; then the middle eye-glass is placed at 

 such a distance from the focus of the first eye-glass, as is the focus of this 

 middle eye-glass; lastly, the nearest eye-glass is placed so nmch distant from the 

 focus of this middle eye-glass, as is the focus of this nearest eye-glass ; and 

 the eye, looking through them all, is placed in the focus of this nearest eye- 

 glass. Therefore, first, one single convex-glass cannot properly be said by it- 

 self to show objects erect or reverse, but in respect of placing of the eye that 

 looks through it. For if the eye be placed nearer to it than the focus of the 

 glass, the objects are erect ; if the eye be placed just in the focus, the objects 

 are neither erect nor reversed, but all in confusion, and between both ; and if 

 the eye be placed further from the glass than the focus, the objects are re- 



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