CONTENTS. 



HISTORICAL SKETCH OF THE TELESCOPE. MEMOIR DIVIDED INTO SIX SECTIONS: 

 1. GRINDING AND POLISHING THE MIRRORS . . . . . . .2 



1. Experiments on a metal speculum. Corrosion by aqua regia ; voltaic grinding . 2 



2. Silvering glow. Finn-unit's and Cimeg's processes ; details of silvering a mirror ; thick- 



ness and durubility of silver films ; their use in daguerreotyping . . 2 



3. Grinding and polishing glass. Division of subject . . . . .6 



a. Peculiarities of glass ; effects of pressure ; effects of heat ; oblique mirrors . 6 

 6. Emery and rouge ; elutriation of emery . . . . . .10 



c. Tools of iron, lead, pitch ; the gauges ; the leaden tool ; the iron tool ; the pitch 



polisher .......... 10 



d. Methods of examination ; two tests, eyepiece and opaque screen ; appearance of 



spherical surface; oblate spheroidal surface; hyperbolic surface; irregular 

 surface; details of tests; atmospheric movements ; correction for parallel rays 

 by measure ; appearances in relief on mirrors . . . . .13 



e. Machines ; Lord Rosse ; Mr. Lassell ; spiral stroke machine ; its construction and 



use ; the foot-power ; method of local corrections ; its advantages and disad- 

 vantages ; machine for local corrections ; description and use . . .19 



4. Eyepieces, plane mirrors, and test objects . . . . . .26 



2. THE TELESCOPE MOUNTING . . . . . . . . .27 



Stationary eyepiece ; method of counterpoising . . . . . .27 



a. The tube ; the mirror support ; air sac ; currents in the tube . . .28 



b. The supporting frame . . . . . . .31 



3. THE CLOCK MOVEMENT .... .... 33 



a. The sliding plateholder ; the frictionless slide . . . . 33 



b. The clepsydra ; the sand-clock ... ... 36 



c. The sun camera . . . . . . . . .40 



4. THE OBSERVATORY .......... 41 



a. The building . . . . . . . . . 41 



b. The dome ; its peculiarities . .... 44 



c. The observer's chair . . . . . . . .45 



5. THE PHOTOGRAPHIC LABORATORY . . . . . . . .46 



a Description of the apartment . . . . . . .46 



6. Photographic processes ; washed plates ; difficulties of celestial photography . 47 



6. THE PHOTOGRAPHIC ENLARGER . . . . . . . .51 



a. Low powers ; use of a concave mirror, its novelty and advantages ; of the making 



of reverses . . . . . . . . .51 



6. High powers ; microscopic photography . . . . . .54 



