97 



A paper was read from G. W. Eoyston-Pigott, M.A., M.D. 

 Cautab., " On High Power Definition with illustrative examples." 



The researches of the author with regard to the defining power 

 of object-glasses of diminutive focus have led him to the conclusion 

 that many objects are falsely represented under object-glasses of 

 from jijth to -^^jyth inch focus. The misrepresentation he attributes 

 to a certain " residuary aberration (chiefly spherical)," which he 

 tliinlfs he detects on all glasses of high power. He applies his 

 views more especially to the most ordinary microscopic test- 

 object, the Podura scale. He states that he has been able to 

 resolve what have hitherto been regarded as minute spines into 

 rows of beads. 



It is well known that under a low power, as 80 or 100, the 

 Podura is remarkable for its wavy markings (these are a safe guide 

 in selecting the scale), aptly compared to " watered silk." It is 

 here that the secret of their cause and nature is to be sought for : 

 hitherto one which has bafiled the most famous glasses of modern 

 times. As a simple fact sometimes leads to a suggestion, view 

 carefully against the light two pieces of silk woven with the finest 

 weft and warp placed one over the other : accordingly as one 

 is lightly stretched more than the other or as the weft of one is 

 inclined more or less to the weft of the other, instantly an endless 

 series of waves are developed by the lines of optical interference, 

 mesh intersecting mesh with infinitely varied eff'ect ; but always 

 waves. Can the waves of the Podura be similarly caused ? 



Raising the power to 200 or 250 and using a side light upon 

 our scale athwart its length, all waviness disappears, and in its 

 place is seen a longitudinal ribhincj, shaded very darkly ; with a 

 less oblique side light, lucid rhomboid chequers glitter brightly : 

 the rhomboidal sides, crossing at acute angles, may be seen with 

 a low power of 500. With 1200 these ribs have divided them- 

 selves into a string of longitudinal beads. But with 2300 they 

 appear to lie in the same plane, and terminate abruptly on the 

 basic membrane : upon focussing for the strings of beads attached 

 to the lower sides the beading appears in the intercostal spaces. 

 The upper beads are best seen either green upon a pink ground 

 or pink upon a greenish ground, which phenomena may possibly 

 arise from the diflerent dispersive powers or refraction of the 

 various structures or the correction of the glasses ; or even more 

 recondite causes. 



When the light is much more oblique, yet achromatic, the beads 

 appear shaded as roughly represented in the diagram, the inter- 

 vening spaces showing fine traces of intersecting lines. 



Using now an adjusting ^-inch at 250, and rotating the scales, 

 some of the most favorable positions, with oblique light, inclined 

 about 15 degrees to the axis of the scale, show a double set of 

 longitudinal lines forming a lattice-work. These lines are the 

 markings existing on the other side of the scale. 



With 300 to 500 the celebrated ." spines " appear, according to 

 the size of the scale, as very dark short tapering marks (like 



VOL. X. — NEW SER. G 



