﻿on Plates whose Surfaces are absolutely Parallel. 491 



the more familiar may be attributed to the great difficulty of 

 preparing artificial surfaces of the necessary accuracy. The 

 demand for equality of thickness is satisfied naturally in 

 plates of mica obtained by cleavage, and again when a layer 

 of water rests upon mercury*. 



There is no difficulty in repeating Haidinger's observation. 

 The transmitted rings are best seen by holding the mica 

 close to the eye (focused for infinity) and immediately in 

 front of a piece of finely ground glass behind which is placed 

 a salted Bunsen flame t- If the mica be very thin, of the 

 kind sold by photographic dealers — perhaps '05 mm. thick, 

 the rings are on too large a scale. Bat if the plate be in- 

 clined to the line of vision, the circular arcs arc well seen 

 and, owing to the enhanced reflexions, exhibit more contrast 

 than is attainable at perpendicular incidence. When it i- 

 desired to examine the complete rings, the plate should be 

 much thicker. I have experimented especially with two 

 plates, "185 mm. and '213 mm. thick, and have observed 

 some novel effects, evidently dependent upon the doable 

 refraction of the mica, hitherto it would seem not taken 

 into account. 



Very cursory observation on these plates, held squarely, 

 showed that with the thinner '185 mm. plate the inner rings 

 were well seen, while with the thicker one they were not. 

 Familiarity with Fabry and Perot's apparatus at once sug- 

 gested that the complication might be due to a double system 

 of rings, corresponding to the two D lines, accidentally co- 

 incident in the first case but interfering with one another in 

 the second. It soon appeared, however, that the duplicity of 

 sodium light was not the cause. The substitution of a helium 

 vacuum-tube for the salted Bunsen made no material difference. 

 And further, calculation showed that the two soda systems 

 would be practically in coincidence in both cases. Thus, if 

 we take as the mean thickness '20 mm. and a refractive 

 index of 1/5, the relative retardation is 2 X 1*5 X '20 or 'GO mm. 

 The wave-lenoth for soda light is 5*9 x lO -4 mm., so that the 

 order of the rings under observation is about *60-r- 5*9 x 10~ 4 , 

 or very near 1000. Now the wave-lengths of the two soda 

 lines differ by about one-thousandth part, and thus the two 

 ring-systems are almost in coincidence. As the thickness 

 increases from '20 mm., the concordance would be lost, but 



* ' Nature/ xlviii. p. 212 (1893) ; ' Scientific Papers,' iv. p. 54. 



•f* According to Prof. Wood's recommendation the salting is best 

 effected with the aid of a piece of asbestos, previously soaked in brine, 

 wrapped round the tube of the Bunsen and forming a prolongation 

 of it. 



