260 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[July, 



THE DIPLEIDOSCOPE. 



Fig. 1. 



Mr. Dent, the eminent clironometer maker of the Strand, had long felt 

 persuaded that the interests of Horology would be greatly promoted, 

 if the public could obtain a cheap, siin|)le, and correct transit-instru- 

 ment, requiring little sr no scientitic knowledge for its right use, and 

 not readily susceptible of injury or derangement. To this end he 

 devoted much time and thought; and, in 1S40, he considered that he 

 had succeeded in inventing an apparatus which, by means of shadom; 

 would produce the desired result. This idea he communicated to 

 Mr. Bloxam, who thereupon informed bim that his own attention had 

 been for some years devoted to the same object, and that he had con- 

 trived an optical arrangement, which, by the agency of a single and 

 double reflection, determined the sun's passage over the meridian 

 with great exactness. When the optical instrument, although com- 

 plicate in its then form, was shown to Mr. Dent, he was immediately 

 struck with the superiority of the contrivance over that which had 

 suggested itself to him : his own method afibrded three observations, 

 but it was attended with the defects and inconvenience which result 

 from the uncertainty of shadows. Convinced that the reflecting planes 

 would elFectually accomplish the desired end, lie entered into an ar- 

 rangement with Mr. Bloxam to undertake their manufacture ; and, 

 after nearly two years' attention on the part of that gentleman, and at 

 great labour and expense on the part of the proposer, the instrument 

 wliich we arc about to describe was perfected, and may now be had 

 at the trifling expense of 2 guineas. 



The instrument possesses great advantages over any other of similar 

 correctness; being exceedingly simple, it is not liable to get out of 

 adjustment or repair, nor does it require any attention beyond that 

 which is, of course, necessary in the first instance, viz. : that it be 

 placed on a level surface, and in the meridian. The observations to 

 be taken afterwards, can be made by any one, although previously 

 unacquainted either with astronomical apparatus or practical astrono- 

 my ; the instrument being as simple as a sun-dial, while it is infinitely 

 more correct, since it gives the time to within a fraction of a second. 

 The utility of possessing an indicator of this kind in addition to the 

 most perfect time-keeper, must be evident ; for, however excellent a 

 clock or watch may be, experience shows how difficult it is to obtain 

 exact time, ybr kngtlmied ptriods,hy any mere mechanical contrivance. 

 To remedy the defect of mechanism, it has been already remarked, 

 that actual observation of the heavenly bodies becomes indispensable; 

 as, without it, the best time-keeper cannot be implicitly depended 

 upon for any considerable interval. On the importance of exactness 

 in this essential matter, it is not necessary to enlarge : it will suffice 

 merely to allude to the inconvenience of missing a railway train. An 

 advantage ;Jso not to be overlooked, is the gratification of knowing, 

 especially in remote parts of the country, tliat you are in possession 



of the true time. Perhaps, then, it is not saying too much to affirm, 

 that a Dipleidoscope should be placed in all country Parsonages, as 

 well as in Railway stations, and government establishments, both at 

 home and abroad. 



By the aid of this new patent meridian-instrument, which is called 

 The Dipleidoscope, any person may obtain correct time with the 

 greatest facility, by an observation either of the transit of the sun over 

 the meridian by day, or of the transit of the stars by night. In the 

 following explanation, however, it is intended, for the sake of consult- 

 ing both brevity and simplicity, to confine the directions to solar ob- 

 servation. 



In the l.mgnage of philosophy, the law which governs the transmis- 

 sion of light is, tli;it the angle of the rays of incidence is equal to the 

 angle of the rays of reflection. In other words, supposing the rays of 

 light proceeding from an object to fall upon a reflecting plane, the eye 

 of the observer must, in order to see the reflected image, be placed at 

 precisely the same angle with regard to the plane, as the rays pro- 

 ceeding from the object to the plane. The rays falling upon the plane 

 from the object are styled "the rays of incidence;" as the r.iys again 

 procnedihg from the plane to the eye are termed the "rays of reflec- 

 tion." Keeping this law or principle in view, let us next consider 

 the construction of the reflecting planes of the instrument in question. 



There are three reflecting planes, Dt, db, and BC, tig. 2. Suppose 

 DC to be so divided that the ray, No. ), hilling on DC, at H, will be re- 

 flected to the eye at 1', and the image of the sun will appear to ad- 

 vance in I he direction from D to c. The ray, No. 2, passing through 

 D c, is reflected from c B, impinges on D B, and reaches the eye in tlie 

 direction 2'. The image of the sun thus formed will appear to move 

 from c towards D, because it has been twice reflected, and thus the 

 two images will approach each other. Suppose the ray No. 1 to have 

 advanced to the position No. 3, and the ray No. 2 to the position No. 

 4 ; it will then be evident that their reflected rays will be in the same 

 direction 3' and 4', and, therefore, that the two images of the sun 

 coincide, as shown by the arrows being in the position of crossing each 

 other, and indicating the instant of apparent noon ; as the rays con- 

 tinue to advance, the images, having passed over each other, will, of 

 course, be seen to separate. 



Fig. 2. 



The following familiar illustration is introduced to further explain 

 the optical construction. When the sun is about setting, it is not un- 

 common to see the rays so reflected from the windows of a whole 

 range of houses, as to convey the idea of a public illumination. While 

 some portions of the sun's rays are thus reflected, other portions pass 

 through the glass into the rooms. The rays thus transmitted (the 

 rays of incidence, as they were styled above) may be thrown at plea- 

 sure in any direction consistent with the range of the sun, by a person 

 within the room, having a looking-glass in his hand : exactly as chiN 

 dren produce what they call a Jack o' lantern. Now if, instead of 

 throwing the rays upon a non-reflecting object (such as the wall, &c.), 

 he were to transfer them to another looking-glass, they would be again 

 reflected from this latter glass. Supposing these two looking-glasses 

 to be placed at an angle of less than 90^, in a manner corresponding 

 with the position of the two silvered planes seen in the instrument, 

 and also shown in the diagram at D B, B c, he can reflect the sun's rays 

 again out of the window. Now, if we imagine the window to repre- 

 sent the outer reflector of the meridian-instrument, its construction is, 



