1886.] 



On a New Form of Stereoscope. 



317 



have generally the same character and range at intervals of half a 

 lunation, and opposite characters at intervals of a quarter of a luna- 

 tion. An expression for the variation at any age of the moon that 

 would satisfy these characteristics would take the form 



/..,(*) co S 2(^i)+/ 5 . 2 (A) sin 2(Jt), 



where h is the hour of the solar day, P the mean period of a lunation, 

 and t the age of the moon, and / C . 2 (X), / s . 2 (^) are solar diurnal varia- 

 tions that are constant for the same season of the year. It was found 

 that although such a formula embraced the bulk of the phenomena, 

 there remained minor characteristics of a systematic kind that found 

 expression only in the extra terms of the formula when extended as 

 follows : — 



cos(^) + / 5 . 1 (A)sin(^)+/ c . 2 (;0 cos 2( 2 p "i)+/ s . 3 (*)sin 2^*) 



Not only does the hypothesis hold good in the different seasons of 

 the year and with respect both to the declination and horizontal force 

 at Bombay, but the variations of the two elements are related to each 

 other in a definite manner ; in the winter season the variations of decli- 

 nation at one age of the moon are similar to those of the horizontal 

 force- at an age of the moon one-eighth of a lunation greater ; and 

 conversely, in the summer and autumn the variations of horizontal 

 force take precedence of those of the declination by one-eighth of a 

 lunation. So far as the means of testing it are available, the hypo- 

 thesis holds also in respect of magnetic variations at Trevandrum. 

 Each term of the formula is symbolical of a definite physical concep- 

 tion, viz., that an otherwise constant variation swells and contracts 

 with a wave-like motion, as the age of the moon increases, between 

 the limits —f(h) and +f(h). The existence of luni-solar variations 

 of the kind described is, so far as the author is aware, brought to 

 light for the first time, by the long series of observations taken at 

 Bombay, and their capability of expression in a compact form which 

 has a definite physical significance cannot, the author thinks, fail to 

 be helpful towards the discovery of the physical conditions that lie 

 behind them. 



IV. " On a New Form of Stereoscope." By A. Stroh. Commu- 

 nicated by Lord Rayleigh, D.C.L., Sec. R.S. Received 

 March 22, 1886. 



Although the late Sir Charles Wheatstone's beautiful invention, 

 the stereoscope, gives the appearance of fall relief or perfect solidity 

 to photographs of objects seen by its aid, the photographs for the 



