222 ANNUAL OF SCIENTIFIC DISCOVERY. 



any equivalent contrivance by which the focal image is measured instead of 

 the focal length ; but, as these optical means are not always at hand, it may 

 perhaps be of some use to explain a mode of measurement practised by my- 

 self very successfully more than twenty years ago. The requisite apparatus, 

 if it can be so termed, will be described in its original simplicity ; a little in- 

 genuity would easily improve it : but even in its first rude trial it was found 

 adequate to its object. Three pieces of cork are perforated by a knitting 

 needle, so as to slide along it. To the centre one is attached, in a vertical 

 position, and with its axis parallel to the knitting-needle, the lens to be meas- 

 ured ; in each of the others is inserted a piece of a sewing-needle, with the 

 point uppermost, and having its length so regulated that a line joining these 

 points would pass, as nearly as may be, through the centre of the lens. The 

 cork discs carrying these needles are then moved backwards and forwards, 



*/ o 



till the inverted image of the one needle's point, formed by rays passing 

 through the lens, is seen coincident and equally distinct with the other 

 needle's point, when both are viewed at once through a tolerably strong 

 magnifier applied to the eye, and directed towards the lens. Then, if the 

 needle-points are sensibly equidistant on each side of the lens, a condition 

 which can be quite sufficiently attained in the course of a few trials, it is evi- 

 dent that they occupy the conjugate foci, and the distance between them be- 

 ing carefully measured with compasses, will be, as a very simple proposition 

 in optics will show, four times the amount of the focal length of the lens for 

 parallel rays. The apparent defect of this method is the uncertainty whether 

 the points, when the image of one is formed close to the other, are equidis- 

 tant from the lens, the setting of which, or its form, unless equally convex 

 on each side, may render actual measurement unsatisfactory. 



ON A TELESCOPE SPECULUM OF SILVERED GLASS. 



The following communication was presented to the British Association, 

 Dublin, by M. Leon Foucault : The astronomical refractor compared with 

 the reflecting telescope of the same dimensions, has always had the advant- 

 age of giving more light ; the pencil of rays which fall on the object-glass pass 

 through it for the most part, and are employed almost entirely in the forma- 

 tion of the image at the focus ; while on the metal mirror a part only of the 

 light is reflected in a converging pencil, which loses still more by a second re- 

 flection being brought back towards the observer. However, as the reflecting 

 telescope is essentially free from aberration of refragability, as the purity of 

 its images depends only on the perfection of a single surface, as with regard 

 to focal length it possesses a greater diameter than the refracting telescope, 

 and thus partly regains the light wasted by reflexions some observers con- 

 tinue to give it the preference, chiefly in England, over the refracting teles- 

 cope for the examination of celestial objects. It is certain that at this mo- 

 ment, and despite the multiplied improvements in the manufacture of large 

 glasses, the most powerful instrument directed towards the heavens is a teles- 

 cope with a metal speculum. The telescope of Lord Rosse is six feet Eng- 

 lish in diameter, and its focal distance is fifty-five feet. Possibly the reflect- 



