TRANSACTIONS OF THE SECTIONS. 37 



that the velocity of the disk's motion decreasing, the top no longer describes a 

 circle, but a continuous spiral line, on which the small ellipse revolves. 



These motions, however complicated they may be, may be gi-aphicaliy shown by 

 holding a blackened paper to the top of the axis of the apparatus, and causing it 

 to approach steadily, when the axis becomes more inclined by the diminution of 

 the velocity. ' 



To do this more easil;^ and with more precision, near the rotating disk is placed 

 a_ support, with a brass frame for holding a sheet of blackened paper, exactly at a 

 right angle to the support. The top of the inclined axis may be brought into slight 

 contact with the blackened surface of the paper by lowering the brass frame on the 

 stand by means of a micrometer-screw, so as to maintain the contact for some time. 

 _ The specimens of curves described by the apparatus show that without any 

 distiu-bing force the top describes a circle. 



If we put a circular disk excentrically on the axis of the apparatus, it still describes 

 a circle, but also an ellipse revohdng on its periphery, whose lenffth of axis de- 

 pends on the weight of the circidar disk fastened to the axis. If the top marks 

 for a longer time, instead of a circle a spiral line is described, with ellipses revolv- 

 ing on it. 



_ Diagrams were exhibited, showing the same curves, but with heavier circular 

 disks on the axis. 



These experiments may be made also by putting on the top of the axis a globule 

 of silvered glass, reflecting the light of the sun, or of a lamp, showing at a con- 

 siderable distance the pretty designs of the nutation curves. It is very instructive 

 to exhibit and explain the complicated phenomena of the luni-solar precession and 

 nutation of the earth's axis by the same apparatus. 



The combined action of the sim and moon's masses on the earth are represented 

 by the small paper sheets put excentrically on the axis of the rotating brass disk 

 of the apparatus. 



The sun and moon's distances from the centre of the earth continually changing, 

 produce the same effect as those circular disks put excentrically upon the axis of 

 the apparatus, and produce an entirely similar motion of the axis of the earth, 

 describing likewise a cone, or a circle on the top of the earth's axis ; and by the 

 changing action of the sun and moon at different distances from the earth, there is 

 produced an additional small elliptical motion, quite similar to those represented 

 m the diagrams exhibited. Similar but still larger elliptical motions are produced 

 in the same manner by the combined and varying action of the sun and earth's 

 masses on the moon, known in astronomy as the precession of the nodes of the 

 moon, and as the nutation and evection of its axis. 



Light. 



Description of a Set of Lenses for the Accurate Correction of Visual Defect. 



By Philip Braham. 

 The lenses shown were piano-spherical and cylindrical. By using piano- instead 

 of double spherical lenses, we are enabled to' add or diminish the power of any 

 given piano-lens to the greatest nicety; so that without multiplying the tools used 

 by the lens-gi-inders, any graduation of focus can be obtained. 



In correcting astigmatic defect, the cylindrical lenses being piano-, and the edges 

 groimd to the same exact diameter as the spherical, they tit together and act as 

 one lens. 



Description of a Parahohidal Reflector for LigJitJiouses, consisting of silvered 

 facets of ground-glass; and of a Differential Holopliote. Bu Thomas 

 Stevenson, F.R.S.E., M.I.C.E. 



The superior advantages of the Dioptric as compared with the Catoptric systems 



