APPARATUS FOR EXHIBITING COLOURS. 285 



The twoglaffes rauft be retained in their pofition by means of* 

 three fcrews ; for, as foon as the pretfure is removed, the/ 

 repel each other with confiderable force ; and, for thi<^ reafon, 

 neither of them ought to be very thin, otherwife they will 

 bend before they are fufficiently near. 



For adjufting the glaiTes to the microfcope, it is convenient This apparatus 

 to fix them in a cylinder of fufficient fize to project beyond the ™ a y be fix ? d ia 

 glatTes and their fcrews, in order that they may be readily crofcope. 

 turned fo as to reflect the light coming from the fpeculum into 

 the direction of the axis of the microfcope : it is obvious that, 

 in this cafe, they muff be fomewhat inclined to the light, fo 

 that the focus of the whole image will never be equally per- 

 fect ; and, inftead of being circular, like the rings themfelves, 

 their images on the fcreen will be oval. In this manner, eight The effeft 

 or ten alternations of colours may eafily be obferved ; but their 

 order and fequence is too complicated to be eafily underilood : 

 for they are really compofed of an infinite number of feries of 

 rings of different magnitude, each feries being formed by each 

 of the gradations of light in the prifmatic fpectrum, which, 

 near the centre, are fufficiently feparate to form diftincl ap- 

 pearances, either alone or in combination ; but, after eight or 

 ten alternations, are loft in the common effect of white light. 

 For, when the glaffes are illuminated by homogeneous light WiCh ^ mo S en * t 

 only, feparated from the reft by the refraction of a prifm, or 

 otherwife, the rings of each colour occupy, together with the 

 dark fpaces, the whole vifible furface, their number being 

 only limited by the power of the eye in perceiving objects fo 

 minute as the external ones become, in confequence of the 

 rapid increafe of the thicknefs of the plate of air near the edges 

 of the curved furface. This circumftance being once under- and fti!I more 

 flood, it is alfo capable of being illuftrated in a manner frill el ^ tjy by a 

 more elegant, by placing a prifm a i'ew feet from the microf- 

 cope, leaving only a narrow line of its furface expofed to the 

 incident rays, and then throwing the rings of colours on it, in 

 fuch a direction that this line fhall pafs through their centre. 

 Care being taken to exclude from the prifmatic fpeclrum thus 

 formed all extraneous light, it exhibits a moft intereiting ana r 

 lyfis of thefe colours ; for the line confifts of portions of the Interefting ef- 

 rings of all poffible gradations of colour, each forming a broken * 

 iine, but not of the fame dimenfions; and, by the prilmatic 

 refraction, all thefe broken lines are feparated and placed pa- 

 rallel 



