E^peyhnint on the Infeclion of Light. r3 



undertaking confidered ns a manufaftory. Thefe enquiries led me to the Doaor him- 

 fdf i from whom I underftand, that all the praftical difficulties are removed, and that 

 the delay which has prevented the philofophical %vorld from being yet fupplied with 

 thefe Inftruments has arlfen merely from the interruption of engagements. And as I 

 have reafon to exped fome communications from him on this fubjea in future, it becomes 

 unneceflary to enter into any of thofe remarks, in this place, which the immediate con- 

 fideration of the fubjeft might fuggeft. 



n. 



A Remarkable Effea of the Infeaion of Light paffnig through Wire Cloth, not yet clearly 



explained. 

 In the fecond volume of the Tranfaaions of the American Philofophical Society, p. 2oi, 

 an optical problem is propofed in a letter from Mr. Hopkinfon to Mr. David Ritten- 

 houfc, who has given a-folution in his anfwer. Mr. Hopkinfon, upon looking through 

 the threads of a filk handkerchief, held clofe before his face, at a diftant lamp, was furprifed 

 to obferve what he thought to be the threads magnified ; and dill more, that thefe fup- 

 pofed threads remained ftationary, notwlthftanding any motion he gave the handkerchief 

 to the right or left. 



Mr. Rittenhoufe, in his remarks on this phenomenon, obferves very juftly, that the 

 appearance, which is that of certain luminous points regularly arranged in right lines 

 croffing each other, cannot confift of any image of the threads of the handkerchief, but 

 muft be certain images of the lamp formed by the infleaion of light paffing near the 

 threads. He appears to have confidered the handkerchief as a very imperfea inftrument^ 

 and therefore too haftily threw it afide for an inftrument confifting of parallel hairs not crofled 

 by others. VVith this he obferved that the line of light or portion of the (ky feen through 

 a window-fhutter very nearly clofed, appeared multiplied, fo as to confifl: of three parallel 

 lines almoft equal in brightnefs, and on each fide four or five others, much fainter, and 

 growing more faint, coloured and indifUna the farther they were from the middleline. 

 When the hairs were thicker, but at the fame diftance apart, or number in the inch, fo as 

 to diniinidi the opening between them, the middle lines were Ms bright, but the others 

 (ironger and more diftina ; and he could count fix on each fide of the middle line feem- 

 ing to be equally djfl:ant from each other, cftimating the diftance from the centre of the 

 one to the centre of the next. In this experiment, the hairs were the 190th of an inch 

 in diameter, and were regulated by paffing them over the threads of a fcrew of 106 turns in 

 the inch : fo that the opening or interval between hair and hair was TJ^th of an inch. Tlie 

 middle line was well defined and colourlefs; the next two were likewife pretty well defined, 

 but fomething broader, having their inner edges tinged with blue and their outer with red. 

 The others were more indiftina, and, confided each of the prifmatic colours in the fame 

 order ; which, by fpreading more and more, feemed to touch each other at the fifth or fi\th 

 line ; but thofe neareft the middle were feparated from each other by very A.\xk lines mucli 

 broader than the bright lines. 



From experiments made by applying this frame of parallel hairs before the objea-glafs of 

 a fmall telcfcopc fuiyiiflicd with a micrometer, he found that, the pofitiun of the two. 



ne.ired 



