CONTENTS. 



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

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



1. Ejri>?riments on a metal in-mlum. Corrosion by aqua rcgia ; voltaic grinding . 2 



2. Silvering glass. Foucault's and Cimeg's processes ; details of silvering a mirror ; thick- 



ness and (lurul)ility of silver films ; their use in daguerreotyping . . 2 



3. Grinding and jtolishing glass. Division of subject . . . . .8 



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

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



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



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



</. 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 fooUpower ; 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 platcholder ; the frictionless slide . . . . .33 



6. 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 



b. 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 



