450 HISTORY OF SCIENCE. 



thin films, of which " Newton's Rings " form a striking example ; for, 

 seven years before Newton's experiments were made, Hooke had 

 published in his " Micrographia " an account of these phenomena 

 and an explanation. He described the coloured films that are ob- 

 tained by dividing a piece of transparent and colourless talc into flakes 

 sufficiently thin, or by blowing glass to the greatest possible tenuity. 

 He remarks that any transparent substance made sufficiently thin will 

 show colours, and he shows that the particular tint depends upon the 

 thinness. In his explanation of these effects he comes so near to the 

 undulatory theory, that Young states that he would himself have earlier 

 been led to that theory if he had previously seen the passage in the 

 " Micrographia " where Hooke thus speaks of the cause of the colours 

 of thin plates : " It is most evident that the reflection from the under 

 or farther side of the body is the principal cause of the production of 

 these colours. Let the ray fall obliquely on the thin plate, part there- 

 fore is reflected back by the first superficies, part refracted to the 

 second surface, where it is reflected and refracted again ; so that after 

 two refractions and one reflection there is propagated a kind of fainter 

 ray, and by reason of the time spent in passing and repassing, this 

 fainter pulse comes behind the former reflected pulse ; so that hereby 

 (the surfaces being so near together that the eye cannot discriminate 

 the two pulses) this confused or duplicated pulse, whose strongest 

 part precedes and whose weakest follows, does produce on the retina 

 the sensation of a yellow. If these surfaces are farther removed 

 asunder the weaker pulse may become coincident with the reflection 

 of the second or next following pulse from the first surface, and lie 

 behind that also, and be coincident with the third, fourth, fifth, sixth, 

 seventh, or eighth ; so that if there be a thin transparent body, that 

 from the greatest thinness necessary to produce colours does by de- 

 grees grow to the greatest thickness, the colours shall be so often re- 

 peated, as the weaker pulse does lose paces with its primary or first 

 pulse, and is coincident with a subsequent pulse." 



Young affirmed that Newton's rings and the colours of thin plates 

 are due to the interference of undulations reflected from both sur- 

 faces of the film. Each ray 

 is in part reflected from the 

 first surface, and in part trans- 

 mitted with refraction, so that 

 this part reaches the second 

 surface, where again it is in part 

 reflected, and this part passing 

 out with another refraction 

 FlG - 201 - through the first surface, 



emerges in a direction per- 

 fectly parallel with the portion of the original ray reflected from the 

 first surface. Thus in Fig. 201, if A B c D represent a thin transpa- 



