1M 



TELESCOPE. 



TELESCOPE, HISTORY OF THE. 



Fir. 10. 



the flexure of each sector it may be divided into three portions of 

 equal weight, and the centre of gravity of each portion being found, 

 it in turn is supported on a pin or knob on one angle of a smaller or 

 secondary iron slab, which in its turn is supported on its centre of 

 gravity by resting on one of the points of the primary triangle. In 

 the case of a six-feet speculum each of these secondary areas may be 

 subdivided into three equal tertiary areas similarly supported on 

 tertiary triangular slabs each supported at its centre of gravity on one 

 angle of a secondary one. In this way the mirror may be conceived as 

 being subdivided into twenty-seven equal areas, each separately sup- 

 ported at its centre of gravity, and thus not liable to bend by its own 

 weight. In practice, however, certain levers are introduced, the 

 action of which does not interfere with the principle of the contrivance. 

 This clever arrangement, however, was defeated by the adhesion or 

 stickage, as it is called, of the metal to its cushion, and this stickage 

 amounted to two tons when the speculum was resting on the bottom 

 of the tube. Hence it has been found necessary to suspend the 

 speculum from above, by means of bands of steel or some other 

 support. 



A reflecting telescope is liable to irregular action from currents of 

 unequally heated air in the tube, which cause remarkable distortions 

 and movements in the images of objects. Sir John Herschel (' Results 

 of Cape Observations ') describes a method of substituting for the tube 

 an open frame-work of iron which gets rid of the objection from 

 counter currents of air. 



Under SPKCULUM some details are given respecting the composition, 

 the grinding, and the polishing of specula. As similar information 

 respecting glass lenses is not given under the article LENS, the deficiency 

 may to some extent be supplied in this place ; the reader desirous of 

 more ample information is referred to the 

 third volume of HoltzapSel's ' Mechanical Mani- 

 pulation,' 1850. The spherical surfaces of 

 lenses are formed by grinding the glass in 

 counterpart metal tools (Jig. Ill), prepared to 

 the proper curvatures with the assistance of 

 grinding and polishing powders. The tools 

 are in pairs, concave and convex, and these are 

 first made to correct each other's errors, which 

 they do on the principle which applies also to 

 the lenses themselves " of the natural tendency 

 of two surfaces which grind each other by 

 equable nibbing over every part to work each other into a spherical 

 concavity and convexity exactly fitting." (HerscheL) Hence it is 

 coinfuiratively easy to form lenses into truly spherical surfaces. 



The concave and the convex tools grind each othei Irue with the 

 assistance of emery and water, although Mr. Ross thinks greater 

 accuracy is attained by using the emery dry. The glass t<,i the lens 

 is brought to the circular form by means of thankt or flat pliers ol 

 soft iron, or to save the material (since good optical glass is more 



:M than gold) selected fragments of glass blocics are softeneu uy 

 to redness, moulded to nearly the required form, and then 

 carefully annealed. Each lens is then coated on one side with a layer ol 

 cement, which is run into a hemispherical mass sufficiently thick to be 

 grasped, so as to form a handle : but if the lens is of large size it is 

 cemented to a metal handle, as a wooden one would swell. The 

 cement is made by mixing wood-ashes with melted pitch. The glass is 

 first rough ground within the metal shell or basin (./'.'/ 11), either with 

 river-sand and water, or coarse emery anc 

 Fig. 11. water, until the surface is brought nearly to 



*^^. -^\ the curve of the shell. The glass is ru " 



I I with large circular strokes, and when 



grinding has been carried far enough, the 



glass is warmed, the cement handle is shifted to the other side, anc 



mding is continued on the second surface of the lens. The 



parallelism of the two sides is obtained by observing that the edge o 



i.-is is of equal thickness all round. The grinding is then con 



I with washed emery, six sizes being used, the last size being 



! powder collected after one hour's subsidence, and which leaves 



so smooth a surface that when the lens is held between the eye anc 



the light it shows a semi-polish. Grinding the lens to the true figure 



ivy the lens, as it is called, is generally done upon a poet witl 

 the concave brass tool, much in the same manner as the concave an " 

 the convex brass tools are made to correct each other. The grinding i 

 continued with each size of emery, until the marks made with th 



us size are removed ; everything being carefully washec 

 between the changes of sizes ; for should a speck of a larger size ge 

 into the work it might make a scratch which would render it necessary 

 to re-commence the grinding. The polishing is performed with th 

 assistance of putty powder, sifted through lawn and enclosed in a bo 

 with :\ lid perforated with small holes; or still better, mixed witl 

 in a corked bottle, which is shaken up every time the powder i 

 to be applied, and allowed to subside for a few seconds. A smal 

 quantity of the water is then taken out with a clean stick, and throwi 



lie polisher, so that only the suspended portions of the putt 

 powder are used. The powder is put upon a piece of thick silk (lute 

 ntring), cut to the width of about seven-eighths the diameter of th 



nid stretched across the middle of the brass tool. The lens i 

 d backwards and forwards in straight lines along the silk, whil 



Fig. 12. 





t the same time it is twisted round in the hand, and also traversed 

 gradually sideways, until the centre of the lens is brought to the edge 

 f the silk, when the direction of the traverse is reversed. For the 

 lost carefully finished lenses, however, a pitch tool is prepared as 

 escribed under SPECULUM. 



Mr. C. Varley has described (' Trans. Society of Arts,' vol. xlix.) a 

 .the for grinding and polishing lenses in which instead of the lower 

 ools being mounted on a fixed post they are mounted on a revolving 

 xis placed vertically. Mr. Grubb, of Dublin, has an apparatus for figur- 

 ng and polishing lenses, which is said to be very successful. For very 

 accurate work, the arrangement shown in fy. 12 

 may be adopted. After grinding in the metal 

 >as in ( fy. 1 1 ) , the lens is attached to the lower end 

 if a vertical rod, of which the upper end termi- 

 lates in a steel ball, working in a cup and fitting 

 accurately, so that every point of the surface of 

 he lens may move in a spherical surface conceu- 

 rie with the steel ball. The rod is grasped by a 

 woollen holder, to prevent the heat of the hand 

 rom elongating the rod. Below the lens is a 

 mall polisher of pitch, spread on brass and 

 jovered with a fine polishing powder mixed 

 with water. The polisher admits of nice 

 adjustment by means of a screw. The rod is 

 worked to and fro as well as circularly, and the 

 ens gradually acquires a perfectly spherical 

 and polished surface, the radius of which can 

 adjusted by lengthening or shortening the 

 rod. In manufactories where large quantities 

 of common lenses are ground and polished, a 

 number are arranged on a convex tool, such as 

 7, 13, or 21 around a central lens, forming 

 what is called a block of lenses. Lenses of 

 medium quality and size are generally ground 

 true and polished seven at a time. 



In forming the object glasses of achromatic 

 telescopes, it is necessary to measure accurately 

 the radii of curvature of the lenses, which are 

 first tried experimentally, and are afterwards 

 made as nearly as possible to the radii obtained 

 by calculation. In order to measure the cur- 

 vature of the grinding tools, Mr. Ross invented 

 an instrument called a tpherometer, a descrip- 

 tion of which will be found in the ' Trans. 

 Society of Arts," (vol. liii.) 



The edges of lenses are made true by grind- 

 ing in a lathe by means of a piece of brass 

 supplied with emery and water ; but in setting 

 the lenses in the lathe for this purpose, it is 

 cemented upon a chuck, and before the cement 

 has set, the lathe is set spinning, and the 

 reflection of a fixed object, such as a candle 

 flame or a window bar is watched, and the 

 lens is adjusted until the image appears to be 



quite stationary, notwithstanding the revolution of the lens. This 

 shows that the axis of the lens and that of the mandril of the lathe 

 coincide and consequently that the lens, when its edge is ground cir- 

 cular, and put in the tube of the instrument, will coincide with the 

 axis of such tube. 



The centering of the lenses of an achromatic telescope, so that all 

 the centres of curvature of their surfaces shall he in one straight line, 

 and that coincide with the common axis of the telescope and its eye- 

 piece, is an adjustment of considerable delicacy. Wollaston's method 

 of doing this is described in the ' Philosophical Transactions' for 

 1822, and depends upon the circumstance that the various images of a 

 candle, of which there are fifteen for a triple object glass, are, if the 

 lenses be exactly centered, all in a right line directed from the candle. 



TELESCOPE, HISTORY OF THE. It has been the fate of 

 almost every instrument by which science has been extended, or the 

 well-being of man promoted, that the precise epoch of its invention, 

 and even the name of the individual to whom the world is indebted for 

 it, are alike unknown. This is particularly the case with the telescope, 

 of which the earliest notices are that it existed in England and in 

 Holland near the end of the 16th or in the beginning of the 17th 

 century. 



There is in Strabo a passage (iii., p. 180, Falconer's ed. ; p. 138, 

 ' Casaub.') in which, speaking of the enlargement of the sun's disc at 

 his rising and setting in the sea, it is stated that the rays (of light) 

 in passing through the vapours which rise from the water, as through 

 tubes, are dilated, and thus cause the apparent to be greater than the 

 real magnitude (of the object) ; and from this it has been inferred 

 (Dutens, ' Kecherches,' &c.), though the inference is probably without 

 foundation, that there then existed tubes furnished with lenses for 

 magnifying objects by refracting the light. It would be needless to 

 make any observations on an inference founded upon an hypothesis so 

 obscurely expressed : the words in Strabo probably signify only that 

 the rays of light might become divergent in passing along the intervals 

 between the [articles of vapour. 



