206 Newton's Letters, Hypothesis and Experiments 



be reflected there ; if double that thickness, that the following 

 rarefied part of the wave, that is, the space between that and 

 the next wave overtake it, there it must be transmitted; if 

 triple the thickness, that the condensed part of the second wave 

 overtake it, there it must be reflected ; and so where the plate 

 is five, seven, or nine times that thickness,- it must be reflected 

 by reason of the third, fourth, or fifth wave overtaking it at 

 the second superficies ; but when it is four, six, or eight times 

 that thickness, that the ray may be overtaken there by the di- 

 lated interval of those waves, it shall be transmitted, and so 

 on; the second superficies being made able or unable to re- 

 flect accordingly as it is condensed or expanded by the waves. 

 For instance, let A H Q represent the superficies of a spheri- 

 cally convex glass laid upon a plain glass, AIR and AIR 

 Q H the thin plano-concave plate of air between them, and 

 BC, DE, FG, HI, &c. thicknesses of that plate or distances 



noidteidr? 

 jcTidlv doidw 



-19tljo 



A&N 



r»i sheld 

 i 



x 



40* 



H f ob 

 Jit 



^■fiw Joqe aril )o 



aq Bflfbira odi 



•yuona hhal 







toa r i9djo arfs 

 t aarealg {qua 1 an 



of the glasses in the arithmetical progression of the numbers 

 1, 2, 3, 4, &c, whereof B C is the distance at which the ray 

 is overtaken by the most condensed part of the first wave; I 

 say the rays incident at B, F, K and O ought to be reflected 

 at C, G, L and P; and those incident at D, H, M and Q 



