6o6 



NA TURE 



[April 30, \\ 



research, for which purpose a siderostat by Foucault, with 

 a silvered mirror i6"3 inches in diameter, is mounted at 

 the northern edge of the platform under a movable cover. 

 Arrangements have, of course, been provided for darken- 

 ing this room at pleasure. In the north wall of the room, 

 and facing the siderostat, is fixed a massive iron frame 

 capable of carrying lenses ranging from the 15-inch 

 object-glass down to that of an ordinary camera, the 

 lenses most frequently used being provided with adjust- 

 able rings so arranged that they need only be centred 

 once for all. For 39 feet of the length of the room three 

 lines of rails are let into the floor, on which travel three 

 iron carriages for the spectroscopes, gratings, or cameras 

 in use, the side rails being intended for apparatus to 

 receive deviated rays. In addition to these a narrow 

 gauge line runs along the centre of the room from end to 

 end, 66 feet. This is intended for long-focus photographs. 

 All these rails are carried by steel beams distinct from 

 those which support the floor. Any one who has been at 

 Dunecht will recollect the comfort with which the most 



that of the 15-inch refractor. The viewing tele- 

 scope is somewhat larger in aperture, to ensure catch- 

 ing the whole of the rays emerging from the prism. The 

 tube of the collimator is made as rigid as possible, 

 and is isolated from the large bronze tube which carries 

 the whole spectroscope. The rays from the great object- 

 glass may be intercepted, just in front of the slit, by a 

 diagonal eye-piece, removable at pleasure, which allows 

 the object to be viewed, and serves also as a finder. 

 Attached to the same draw-tube is a second prism for 

 throwing the light from any artificial source upon 

 the slit. Only one prism can be used at a time, 

 but it can be readily exchanged for another, with- 

 out disturbing the adjustments. The prism is carried 

 by a divided circle, so that its exact position is 

 always known. The long rod shown in the figure rotates 

 the prism ; the shorter one moves the viewing telescope, 

 the position of which may be read either by two opposite 

 microscopes, or by a long reading microscope (not visible 

 in the figure), carried close down to the observer's eye. 



6 in Refractor. 



24-iiich Siderostat. 15-inch Refractor. 



Reflector. 12-inch Optical Room. 



Reflector. Library. 



Fig. I. — Royal Observatory, Edinburgh, from the South-west. 



delicate solar work could be carried out in the optical 

 room there, of which the room at Edinburgh is a copy. 



In the east tower is mounted the 15-inch Dunecht re- 

 fractor. Amongst the adjuncts to this instrument may be 

 mentioned the large stellar spectroscope made by T. Cooke 

 and Sons (Fig. 2). As this spectroscope has not yet been I 

 described, the following particulars may be of interest. It j 

 is provided with three prisms : (i ) one of 60° by the makers, | 

 giving a dispersion of 5^ 7' from A to H, and capable of | 

 separating the lines 488*81 and 488*84 of Angstrom's map. 

 It shows also 43 lines between B and C. It was with 

 this prism that D3 and another helium line, 487*6 mmm. 

 + , were detected in the Great Nebula of Orion at 

 Dunecht in the winter of 1886-87 (see J/<?;7////y Notices, 

 vol. xlviii. p. 360). (2) A large compound prism by Sir H. 

 Grubb, with nearly twice the dispersion of the prism first 

 mentioned. (3) A Merz prism with an angle of 20°, 

 intended for use on the fainter stars. The collimator 

 has a focal length of 24 inches, and an aperture 

 of 2 inches, the ratio being 12 to i, the same as 



NO. 1383, VOL. 53] 



The pointer in the field of view^ is illuminated by mono- 

 chromatic light of any desired colour or intensity, pro- 

 duced by a small direct-vision spectroscope, on the plan 

 devised by Prof Smyth. No detail has been omitted 

 that could contribute to the accuracy of the observations, 

 such as focusmg scales for collimator and viewing tele- 

 scopes, eclipsing screens in the field of view as well as in 

 front of the slit. A thermometer shows the temperature of 

 the inside of the prism box. It will be seen from Fig. 2 

 that measures can be made on either side of the axial line 

 by simply turning the viewing telescope and rotating the 

 prism into the proper position. 



It would be scarcely possible to enumerate the 

 various minor instruments, but the following may be 

 mentioned. Two heliostats by Duboscq and Browning ; 

 photo-measuring instrument by Grubb ; dividing engine 

 by Dumoulin Froment, of 64 cm. range, showing the 

 thousandth part of a millimetre ; delicate Oertling balance 

 and weights ; standard metre a o' by Dumoulin Froment ; 

 standard yard by Simms ; two excellent spherometers by 



