16 Transactions of the Society. 



ness about to be related. The question arose, Is it jDOSsible to 

 estimate a bright space between two spider hues when total separa- 

 tion is only the eight millionths of an inch, the lines themselves 

 being the SUOOth and the 70U0th of an inch respectively, and 

 reduced in the miniature thirty-eight times ? Beducing the numbers 

 to decimals, if S be the space reckoned between the centres of the 

 spider lines, it is evident if t and t' be the spider lines in diameters, 

 and X be the required interval (see Fig. 5), 



The value of S was found by carefully measuring the movement of 

 the micrometer = to t oo ? which just brought the bright separating 

 interval into view. Therefore we have the required size of interval 

 (considering it diminished thirty-eight times), 



3 1111 



38 X 10000 2 38.8000 2 38.7000 



= 0-00000789 - 0-00000164 - 0-00000187 



= 0-00000789 



- 0-00000351 



•00000138 

 23W0^^"^'y' 



or about half the interval between the centres of the wires.* 



The astounding sight of wires or webs separated by an interval 

 of light less than the two hundred thousandth of an inch can only 

 be explained by the light being subdued. Indifferent glasses cause 

 diffraction images, besides clouding over the view with residuary 

 spherical aberration much more difficult of cure than the colour. 

 Without this interval — I may say, this extraordinary interval — one 

 might conclude the webs are in some mysterious manner enlarged 

 in the miniature beyond the calculated value. And so they are in 

 poor glasses ; for the image appears blurred — swelled, as it were — 

 or adumbrated. But now the lovely precision of definition witnessed 

 in high-class glasses, not only of the webs, but of dust on them 

 and specks on the lamp-glass, precludes any suspicion, in face of 

 this interval, of the enlargement of the lines encroaching much 

 upon its dimensions. Besides all this, as the webs pass and repass 



* Putting the decimals into fractions, 



_ 1 1 J , _ 1 



127000' ^ 610000' ^ 532000 



The abo.ve calculation, it must be remembered, refers to the effect of the 



micrometer screw diminislied thirty-«eight times by the Powell and Lealand \ 

 best immersion. 



