248 



THE POPULAR EDUCATOR. 



dressed stones, side by side, so as to form a square, each stone 

 weighing moro than 10 cwt. On these is built up a hexagonal 

 column 5 fee J J G inches in diameter, each layer of which is com- 

 posed of six large cut stones, the whole being finished with a 

 Yorkshire landing 4 inches thick, on which rest the levelling 

 screws of the equatorial. The height from the gravel bed to the 

 levelling screws is about 17 feet, and the pillar contains about 

 500 cubic feet of cut stone. 



The object-glass of this telescope was made by Troughton 

 and Simms out of tho same material from which they manu- 

 factured the largo Greenwich transit and the achromatic at the 

 Cape. The three glasses are all of the same size, viz., 8 inches 

 clear aperture, the focal length of tho Stonyhurst glass being 

 11 feet G inches. A silver circle, 2 feet 2 inches in diameter, is 

 fastened to the central tube of the telescope, and this gives the 

 declination to within 15". Tho fixings of this circle are by Gary. 



At the lower end of the poiar axis are a num/'iei 1 of concentric 

 circles, which render the working of the instrument very effec- 

 tive. Two of these circles aro graduated on silver, and read to 

 within 1". One is immovably attached to the polar axis, and 

 therefore, moving with the telescope, gives the hour angle ; the 

 other is perfectly free, but may bo clamped on to the driving 



Two observing-chairs and an observing-ladder were aiso 

 provided for the equatorial ; and a new object-glass was con- 

 structed about the same time, as the old one was not considered 

 worthy of its perfect mounting. 



The equatorial stands in a circular room twenty -four feet 

 in diameter. The walls of this room are a little over seven 

 feet in height, surmounted by a revolving dome, the shape 

 of which is that of a hemisphere somewhat shortened. The 

 dome runs on twelve 6-inch rollers set in a ring, the rollers 

 working freely between an upper and lower surface of railway 

 metal. The flange of the upper railway metal is bolted to the 

 sole plate of the dome. The framework of the dome is of pitch 

 pine, and out of the sole plate circular ribs converge to the zenith. 

 The frame is stiffened with a reticulation of hoop iron, and 

 covered with j^-inch boards of pitch pine. The outside is pro- 

 tected by a covering of painted canvas, and the internal finish 

 is of American cloth of a dark colour. This dome is moved by 

 a spur-wheel and pinion gearing into a segmented rack resting 

 on the wall plate. The wheel is actuated by a chain. The 

 whole is so placed that the chain is always directly opposite the 

 shutter, and therefore generally close by the hand of the observer. 

 The shutter, 4 feet 6 inches wide, consists merely of a frame of 



FHsr.6. 



s^iniMw 



clock when required, and then, if its fixed vernier be set to 

 sidereal time, the vernier moving with the telescope will give 

 the right ascension of any object in the field of view. The other 

 concentric circles are for the clamps, which fasten either the 

 telescope or the right ascension circle, or both, to the clock- 

 work when necessary ; also for the slow-motion screw, and for the 

 brass wire-rope, which supports a weight suspended within the 

 pier of the instrument. This weight is the motive power of the 

 telescope, tending always to pull it round in the direction of the 

 apparent motion of the heavenly sphere, whilst the clock, con- 

 nected with it by means of an endless screw, serves to regulate 

 the motion. This clock is perhaps the most perfect poriion of 

 the mounting, and is considered a most excellent piece of 

 engineering. All its parts are double; and as it requires winding- 

 up only every six hours, and moves very steadily, you may direct 

 the telescope to any particular object, and then leave the ob- 

 servatory, feeling confident that you will find the object still in 

 the field of view, if you return in the course of a few hours. 



At the eye end of the telescope is a position circle 6 inches in 

 diameter, which reads to 30", and the instrument is well pro- 

 vided with eye-pieces. These consist of several comet-seekers, 

 a spider-line micrometer, two double-image micrometers, etc. ; 

 the set of powers range from 100 to 750, but several of the 

 eye-pieces aro of a lower magnifying power. A direct-vision 

 spectroscope was fitted to the instrument by Browning a few 

 years ago, and Troughton and Sirnrns about the same time 

 mounted six of Hof mann's compound prisms for a large spectro- 

 scope on Mr. Huggins' new model. 



pitch pine, covered outside with painted canvas, and inside with 

 American cloth ; it is worked by pulleys and level gearing. 



Adjoining the central dome are two rooms, one containing a 

 transit by Cary ; the other fitted up as a library, and about to 

 be used for spectroscopic researches. 



Underneath the building are ceiled cellars, in which a hot- water 

 apparatus has been fitted up for the protection of the instruments 

 in damp weather ; a double 2-inch pipe passes round the equa- 

 torial room, and forms a coil in each of the adjoining rooms. The 

 large instrument is also Well lit up with gas, the pipes passing 

 through the iron pier : tho light used is thus under oomplete 

 control. 



THE MICROSCOPE. 



The microscope does not differ essentially from the telescope, 

 for, as in the latter instrument, a spectral image is formed in 

 the tube, which is subsequently magnified, so in the compound 

 microscope an image is formed, and again magnified by using 

 another lens called the eye-piece. 



Dr. Carpenter's drawing of the compound microscope (Fig. 

 4) is abovit the best that can be given, in which the eye-piece of 

 Huygens consists of two plano-convex lenses (E E, F F), with 

 their convex surfaces towards the object at o, viewed first 

 through an achromatic glass called the object-glass. 



A small compound microscope, such as that represented in 

 Fig. 5, can be purchased for a few shillings. Microscopes of 

 this description, though of little value as scientific instruments, 

 are well adapted to amuse children. 



If a little more money is expended, of course the compound 



