OPTICAL INSTRUMENTS. 



31 



these faults are visible to the eye, t the lens 

 is called curdled. If we are desirous of 

 procuring; an uniform and perfect sur- 

 face, the polishing tool must be homo- 

 geneous, and the best material for its 

 formation is good clean bees wax, hard- 

 ened by the addition of red sulphate of 

 iron, dry and finely washed. This com- 

 position, when of the proper temper, is 

 melted over the brass tool ; and when 

 cold can be turned to the required 

 curve. The advantage of this improve- 

 ment, besides its uniformity, is that 

 should any hard scratching particles 

 insinuate themselves between the tool 

 and glasses, they sink and are imbedded 

 in the wax, and thus their injurious 

 effects are prevented. The polish of 

 lenses made in this manner is clear and 

 defined when examined by a micro- 

 scope ; when the shadow of a bar is 

 brought across them. This method is 

 now employed by one of the first opti- 

 cians in the metropolis.* 



Centering of Lenses. The centering 

 of lenses for accurate instruments is of 

 great importance, more especially for 

 the object-glasses of achromatic tele- 

 scopes. Different opticians employ their 

 own methods, but one of the best is 

 done by reflection : let the lens to be 

 centered be cemented on to a brass 

 chuck, having the middle turned away 

 so as not to touch the lens, but near the 

 edge, which will be hid when mounted ; 

 this rim is very accurately turned flat 

 where it is to touch the glass. When the 

 chuck and cement is warm it is made to 

 revolve rapidly: while in motion a lighted 

 candle is brought before it and its re- 

 flected image attentively watched. If this 

 image has any motion, the lens is not 

 flat or central : a piece of soft wood 

 must therefore be applied to it in the 

 manner of a turning tool, till such time 

 as the light becomes stationary. When 

 the whole has cooled, the edges of the 

 lens must be turned by a diamond, or 

 ground with emery. This method of 

 centering and adjusting object-glasses 

 by their reflected images, was laid before 

 the public by Dr. Wollaston, and has 

 been used by our first opticians for a 

 considerable time. 



' * The method of grinding and polishing lenses 

 from diamonds for microscopes, by Mr. Pritchard, 

 will be found in the Quarterly Journal of the Royal 

 Institution, vol. ii, page 14, new series. 



Concluding Remarks on Telescopes. 



(48.) The applications of the telescope to 

 the purposes of man are so numerous, 

 that their details would far exceed the 

 boundaries of our treatise. Amongst 

 its principal uses, however, besides 

 those accompanying the descriptions of 

 the various modifications of that in- 

 strument, may be enumerated the fol- 

 lowing : The accurate determination of 

 the longitude of the various places on 

 the earth's surface, is ascertained by the 

 telescope, by observing the immersions 

 and emersions of the four satellites of 

 the planet Jupiter ; from thence, by the 

 aid of a good chronometer, with the time 

 of any known place, the situation of the 

 unknown spot is determined. Before 

 the invention of the telescope, naviga- 

 tors were compelled to keep within sight 

 of the coast in sailing from one country 

 to another, and thus were often endan- 

 gered while passing an hostile or rocky 

 shore ; by the assistance of this instru- 

 ment, the voyage is made direct to the 

 intended place without fear or danger. 



To the astronomer the telescope is his 

 principal and most important guide. It 

 enables him to determine, with precision, 

 the transits of the planets and stars 

 across the meridian. The computation 

 of astronomical and nautical tables, to 

 determine the revolution of the planets on 

 their axes, and their relative polar, and 

 equatorial diameters, is derived from ob- 

 servations by the telescope. We are by 

 this instrument enabled to discover the 

 analogy between the laws which govern 

 the motions of the planets and those of our 

 earth ; their parallax, and from thence 

 their distances. The aberration of light 

 and the motions of the siderial systems 

 in space, unfold wonders which must 

 excite the imagination of the most pro- 

 found philosophers in the highest pos- 

 sible degree. The harmony and sim- 

 plicity displayed in such immense 

 worlds prove the design and wisdom by 

 which they were created ; and the won- 

 derful facts thus ascertained raise the 

 most ordinary mind up to a sublime 

 contemplation of the great Creator. 



In surveying of land, the telescope is 

 highly useful, and for this purpose is 

 mounted on a stand, with an horizontal 

 and vertical motion registering by divi- 

 sions the degrees and minutes of inclina- 

 tion or position of the instrument. For 

 the more accurate reading o^these divi- 



