CONSTRUCTION OF LARGE TELESCOPE LENSES. 167 



lightkig. There are now employed in this industry almnt 050 i)er- 

 sons, and the vahie of the yearly outiHit reaches ;),000,000 marks. 



AVe are now })re|)ared to tra^'e to the furthei- staijes in the prej)ara- 

 tion of a great ohjective, and the attention of the reader is invited to 

 the optical workshop. Here the glass disks are first ground and 

 polished on hoth sides preparatory to a thorough testing. Vov this 

 purpose there is a machine with a vertical spindle carrying an iron 

 plate. 



Upon this plate the glass disks are in their turn cemente(l with 

 pitch, and above is a second iron plate, the gi-inder, i)rovided with a 

 spindle in the center. By means of this spindle the grinding })late is 

 shoved hither and thither over the glass disk by machinery. The 

 grinding material is emery powder and water. After the rough 

 grinding is done the rough polishing on the same macliin.e follows 

 similarly, excepting that the grinding tool is replaced by a cloth- 

 covered polishing tool, covered with rouge instead of emery. 



After this preliminary work, a careful investigation of the disks is 

 made in the laboratory by the aid of the microscope and i)olarization 

 apparatus. If the objective is good it nnist api)ear bright in the 

 polariscope, with the exception of being nuxrked bv a regular dark 

 cross. If an irregular cross is seen or, in certain conditions, brightly 

 colored figures of various shapes, the disk must be returned to the 

 glassworks to be remelted and cooled. 



In case of a satisfactory outcome of these tests small pieces are cut 

 off and prisms are pre})ared from them, whose refractive indices are 

 determined by means of the spectrometer. ITi)on these measurements 

 are based the accurate computation of the objective — that is to say, 

 the determination of the four radii of curvature and the thickness — 

 a tedious piece of work which requires repeated independent checking. 



After this begins the real preparation of the objective lenses, one 

 of which is to be ground conca^'e, the other con^^ex, on the same ma- 

 chine which was used in the rough grinding. This present procedure 

 is similar to that already described, except that grinding tools o])po- 

 site in curvature to the lenses and made of iron, brass, or glass are 

 fed with finer and finer emery powder as the work approaches its 

 finish. 



Since everything depends on the proj^er guiding of the grinding 

 tool to obtain the regular spherical surfaces, the operating of the 

 machine demands great experience and care, and the work recpiires 

 frequent testing by .the application of the spherometer. When finally 

 the right curvature is reached, after many days of work, repealed and 

 accurate testing of the lens is made l)y the Top])ler '' Schlieren- 

 methode " for small errors, nonhomogeneity, and other faults. 



The fine-ground lens is now put upon a lathe and centered by means 

 of a fine adjusting crane. This centering consists of shifting the 



