1837.] Mr W. N. Shaw, Some Laboratory Notes. 99 
diately in front of the object-glass of the telescope. The slit was 
sufficiently long to extend right across the object-glass, and was 
set vertical. At a fixed distance from the telescope a piece ot 
wire-gauze of fine mesh was set up with its wires horizontal 
and vertical; behind it was a ground-glass plate illuminated by a 
sodium flame. The telescope was then focussed on the wire-gauze, 
the slit being set wide open. If the distance was not too great a 
well-defined image of the gauze was obtained. The size of the slit 
was then gradually reduced by turning its screw until it became so: 
narrow that there was not sufficient aperture for the resolution of 
vertical lines (those parallel to the slit), The appearance then 
Distance of POO EUR FERC TagS Calculated 
MeSINOr ee of width of slit by Mean width eR TeT 
ere rater f wedge for positions of reduced ate fe 
pane: t lei 3 e disappearance of the to inches. a ae a 
De: vertical lines. as 
31°5 wires : 250, 249, 250, 247, : = 
folin, |} 76 im || 256, 247 f 004 ey 
310, 362, 365, 370, : 
: ay | a } 085 5858 
a ED = 5 460, 468, 475, 470 112 5960 
= ' if; 438, 445, 437, 450 , 
Corot | 765 ¢,, { Car. } 102 5640 
P 116-5 ,, | 680, 695, 700, 685 | +164 5959 
presented in the telescope was a field covered by horizontal wires 
only. The slit was adjusted to the position at which the vertical 
wires were just no longer resolved, and its width was then 
measured by means of a graduated wedge. The position proved to 
be fairly definite and easily recognisable. The slit must be nar- 
rowed until all trace of vertical lines is gone; the tendency seemed 
to be to judge them as disappeared before the slit was sufficiently 
narrow. To verify Lord Rayleigh’s theory with regard to the re- 
solving power, the measurements required are as described in the 
paper mentioned, viz. / the distance of the gauze from the object- 
glass, d the distance between corresponding parts of consecutive 
wires of the gauze, and A the width of the slit; from these the 
1 At this distance the want of uniformity in the mesh of the gauze was clearly 
apparent, and made observations uncertain. 
