Soltir SpeElrum of uriromnnn BriUuwcy. 15 



parts of the lines about the angle, cannot produce an image fo clear and definite as the 

 fides. The images are farther afunder the finer the cloth, becaufe the nearnefs of the threads 

 increafes the inflexion, infomuch that when the interval is about ^g^th of an inch there 

 will be no central image *. As the images are produced by an equal number of clafies 

 of pencils parallel to each other, any motion of the cloth either parallel or perpendicular to 

 its own plane before the objcdl; glafs will not affecb them, becaufe they come to the objecl- 

 lens ui'.der the fame circumftances of parallelifm. But when the deflections were made 

 by the cloth between the objecl-lens and its focus, the pofitions or diftance of the images 

 were affefled more or lefs upon the principle of the prifmatic micrometer. And, ladly, 

 the microfcope does not (hew this effeif, becaufe the rays from the object, not being parallel, 

 are not uniformly deflefted, and have no virtual foci. 



1 was well fatisfied at firft with this theory, and ftill think the greateft part will, 

 upon further examination, prove to be well founded. But upon repeating the experi- 

 ment with a piece of new cloth made of brafs wire about fifty-five threads in the inch, 

 the wire itfelf being by elUmate about the rl'^ih of an inch thick, other lights became vifi- 

 ble, the caufe of which does not appear fo obvious. Befides the fquare, confiding of nine 

 lights, there were others much fainter, more coloured, and elongated, as in t ig. 1 1. In order 

 to difcern thefe to the greatell advantage, the fmall focal image of the fun was viewed in 

 a concave metallic reflector of two inches focus, by means of the achromatic perfpeclive 

 before mentioned, at the diftance of nine feet, with a power of only eight f. All the 

 images, except that in the centre, were coloured fpedlra, extremely brilliant and beautiful ; 

 the original nine, however, ftill retaining their pre-eminence. They were fainter and more 

 elongated, the more remote from the centre to which they all pointed; that is to fay, an ima- 

 ginary line drawn through the middle of all the colours cf any one of the fpeclra, would have 

 interfered the central colourL-fs fun. Neverthelefs their arrangement with regard to each 

 ether was not radial, but in lines forming prolongations of the fides of the middle fquare, 

 as in the figure of the cloth : thefe lines wee parallel to the threads. The interval be- 

 tween each range of images, meafured from the centre, was greater the farther ofT, but not 

 regularly increafing ; thofe near the middle fquare being almoft equal. Seven ranges were 

 counted beyond the fquare on each fide, and a very long triple line of whitilh faint light 

 extended ftill fartlier. There were alfo a great number of much fainter images irregularly 

 ft;ittcrcd in the angular fpaces between the crcfs, all pointing to the centre, and pro- 

 duced, as I conjectured, by irregularities in the cloth. 



As the diftance of the cloth before the objeit-glafs did not affed the pencils of light, 

 it readily occurred, that by inclining its plane to the axis of vifion in fuch a manner as 

 that one fet of the wires might ftill remain perpendicular to that axis, the other fet 

 would apparently crowd together, and diminilh the openings through which the rays were 

 to pafs. In this manner the cloth might be made, in efFed, of any required finenefs. When 

 the crofs wire or weft of the cloth was inclined to the axis, that part of the crofs in 

 Fig. 1 1 which corrcfponded with the dire£tion of the inclined wires, had the intervals of 

 its fpeftra enlarged, at the fame time that they became broader and more colourcl, but 

 fainter, and in particular the central colourkfs image nearly vaniflied. I have no doubt 



• Newton's Optics, III, obf. 6. 



^ This was for the fake of light, as tlic chamber was not darkened. The ohjcft-g!afs of 9 inches focus 

 inJ I inch aperture bears a magnifying power of fifty, wiilioiit producing any colour. 



but 



