66 Optical Delusion.

ble time for the benefit of the public. It must be acknowledgedj however, that isolated cases occur~ though few and /:ar between, where no complaints of this nature can be made~ and where ihir play has been allowed to have had full scope. T h e ' r r o n Church steepl% Edinburgh, erected, in 1828~ by the architects Messrs. R. and R. Dickson, may be mentioned as an instance of lair competition; the choice of the plan reflects the highest credit upon the then magis- trates of the city~ who made their election from a great number of designs. Considering its cost, this steeple~ for aptness, originality, and picturesque beauty, can scarcely be surpassed in any country. It would not be easy to cite many instances of the same kind in Scot- land, but we may meution aoother~ viz: Burns ~ monument at Ayr, by Thomas Hamilton, Esq., of Edinburgh. This is an exquisite gem of Grecian architecture, of which school its tasteful architect is a dis- tinguished disciple. Finally, it is evident that the grievance here complained of, and which calls so loudly for redress~ is in no way amenable to the civil law, unless in such a case as we have before hinted at, viz: where the judges do not abide by the advertised terms of the competition. Even in such eases, we are not sure how fiar they lay themselves open to have their proceedings legally called in question, so that an appeal can only be made to the moral rectitude of society; and we have no doubt that the evil only requires to be thirly exposed~ to b% in tim% totally eradicated.

Edinburgh, October, 1834.

¶ Optical Delusion. It may not be uninteresting to the readers of your valuable journal,

to be informed of a mechanical method which I have lately discover- ed, that enables the eye to distinguish objects when moving at great velocities.

Having conceived it probable that figures revolving on axes at right angles to their planes might be distinctly see% I constructed an ap- paratus to determine the point~ a short description and sketch of which I give you on the other side.

A drawing of Saturn and his belt~ about five inches diameter~ was then made, and caused to revolve at the rate of 2.500 times a minut% when the figure was found to be almost as distinctly visible as when at rest.

Other drawings were then tried, of a human figure~ a toothed wheel, the number 1835~ the letters R,R, &c., all of which were as distinguishable at that velocity as the first figure was.

It appears to me very likely~ that the same results would be found if the velocity were increased to 10~000~ or even 20,000, revolutions per minute. I am constructing another apparatus~ founded on the same principle, which will enable me to put the correctness of this anticipation to the test of experiment.

~ICHARD ROBERTS. Manchester, Jan. 19, 1835.

C o m p e n s a t i n g P e n d u l u m . 67

Fig. ], is an end elevation of the apparatus; fig. 2, is a sectional representalion in the same direction; and fig. $, is an elevation taken transversely; a, a, are two frame sides of wood, screwed to the base by supported by the spur pieces c, c, c, and connected at top by the piece d. The main shaft, e, mounted in the slandards~ carries a cast iron disc,fi supported by twelve spokes, g, g, g, of wood~ and to these spokes are nailed segments~ h~ h~ of" thin pasteboard.

d ~ ~ I r . . . . . l~ . +l,v+J. +~'_ ~. ~ 3"dt-'l dlo .... , .~

• - - + " h _ " t -J ,

- ' < S l 7,_+',- !,!

//+ In one of the segments a hole, "/, is cut~ one inch long by one and

a quarter inch broad~ and opposite to it another hole is made in the frame sider to which the spectator's eye is to be applied;j, is a small shaft, supported by one of the frame sides, and by a piece attached to d; and k is a pulley~ two inches in diameter, to be driven by aband from the pulley l, of twelve inches diameter on the main shaft; m is a strap pulley, by which motion from any first mover may be commu- nicated to the discs; n is a winch handle, of seven inches radius~ by which 1000 or 1200 revolutions per minute may be given to the small dis% o, on the shaft, j , the centre of which is opposite to that of the eye.hole in the frame side, The small disc is of wood, and to it the figures to be viewed were attached. It will be evident that, if the large disc be set in motion, the small disc will be made to re- volve at about six times the velocity of the large one ; and as the centre of the eye-hole is about two t~et two inches from the centre of the disc, the figure on the small disc will peribrm only the one hundred and ninth part of a revolution each time that it comes under the view, and will appear to be at rest.

Owing to the difficulty of making pulleys work with unvarying re- lation to each other, a kind of jumping appearance was produced, from a change of position in the figure; to obviate this~ I used toothed wheels of six to on% instead of pulleys, anti then the figure ceased to change its position. [Lond. Journ.

¶ Compensating Penduhtm. Annexed is a sketch of a compensating pendulum, ~he idea of which

was suggested by the description of a lever pendulum~ by Captain Forma% in your Magazine for last month.