Method of con- the infpcftlon of the Society. The outer fiiles of It are of 

 port-fo"k)$r^* mahogany, which, beaded, forms the out edge in a neat man-» 

 ner. Its fimple formation, its lightnefs, and its firm flatnefs, 

 mil ft be obvious to every one; and I am of opinion the Society, 

 w#|.l obtain the thanks of all c(^lle6lorss and artifts (if they think 

 it worthy of inferlion in the Volume of their Tranfaftions) by 

 making (his iimple matter publicly known. For my own part, 

 I (hall be highly gratified in having contributed to the comfort 

 of artifts and coilcdlors, in preferving their valuable refearches* 

 I am. Sir, 



Your obedient Servant, 



JAMES M ALTON. 

 Norton-Jlreet, June 25, 1802. 



Reference to the Engraving, Plate VIII. 



Fig. 4. One of the fides of the frame for a port-folio, the 

 tlimenfions four feet by three: it may be made of deal or fit 

 wood. The ftiles, four inches wide by half an inch thick ;' 

 the middle upright ftile to be of the fame width. It fliould 

 have angle pieces within the corners, to keep them firm. 



Fig. 3. Shows a fide of another frame, where much larger 

 dimenfions are required; it fliould then have two upright ftiles, 

 and a middle longitudinal ftile, all within the frame, and angle 

 -pieces at the corners. U it is made of mahogany, inftead of 

 deal, the ftiles may be reduced nearly one-fourth in breadth and 

 thicknefs; the wood, in either cafe, fliould be well feafoned, 

 that it may not be liable to warp. 



XVII. 



Experiments and Calculations relative to Phyfical Optics, By 

 Thomas Young, M. D. V. R. S. From the PhilofophiccA 

 TranfaSiions for 1804-. 



(Concluded from p. 64.) 



g^ Exper. 2. 1 HE crefted fringes defcribed by the ingenious 

 like Wife the and accurate Grimaldi, afford an elegant variation of the 

 crefted fringes ' preceding experiment, and an intereftine example of a calcu- 

 of Grimaldi. J . , , -, ttt. n j • r j 1 1 



lation grounded on it. When a Ihadovv is formed by an ob- 

 ject which has a rectangular termination, befides the ufual ex- 

 ternal 



