224 Dr Royston-Pigott, On a neiu method of [Dec. 2, 



go^QQtli and y^jy^th in diameter, carefully separated by the micro- 

 meter screw three divisions, i.e. yoooo^^^ of an inch when diminished 

 38 times only. 



This interval represents the half-diameter of each line t, t' plus 

 the space intervening between them, cc, so that 



2 '' ' 2 ^ -f -t — 10000- 



0000300 



- 000134 



From which x = 00003 - 000134 = 0000166 



Then the miniature being 38 times smaller : — 



Apparent interval ^ = 0-00000437= 23 0*000 ^^ nearly, the minia- 

 ture spider-lines being each 



1 1 



38 X 8000 38 x 7000 ' 



or ^^000 and aWow respectively. 



If now, for the sake of argument, we suppose the miniature 

 lines slightly thickened, then the bright interval between them 

 would be proportionably less than 230^000 ^^' ^ thing which cannot 

 very well be deemed probable. The objection then that in the 

 miniature gossamers we do not see a miniature reduced in the 

 same proportion is disposed of because the lucid interval by such 

 thickening would become smaller than it is possible to be seen 

 according to the diffraction theory. Besides this, it may be added 

 that young, acute sight distinguishes a much smaller interval than 

 that descried by the writer. On another occasion when the 

 miniature was dimished 50 times the visible interval gave five 

 divisions instead of three. But I am not sure that the "back 

 lash " and strain on the micrometer- screw were equable as those 

 of the first example : besides this the utmost care must be taken 

 to ensure centricity of the " wires " to avoid visual parallax, which 

 is sensible in excentric portions of the field of view. 



The important formula of Helmholtz, it should be remarked, is 

 totally independent of the number of glasses employed or the 

 power employed. It merely states the conditions upon which a 

 destructive overlapping of brilliant diffractions will totally obscure 

 contiguous lines or disks. It would appear to follow from these 

 circumstances, that, whatever image, formed of the various lenses, 

 is examined instrumentally, the same diffractive obliterations must 

 occur — whether therefore the final image at the eye-piece be 

 formed by a series of miniatures in order, and then magnified i;p, 

 or by sufliciently small bright objects without miniatures at all, 

 the law equally holds good. 



