OONSTRUOTION OF LARGE TELESCOPE LENSES, 1()7 



Ii<;htin<j:. Thciv aiv now eniplovod in (his industry nboul OHO per- 

 sons, and the value of tlie yearly output r(>aclies ;^,0()(),()()() marks. 



AAV ai"e now prepared to trace to the fui'thei* staijes in the ])repara- 

 tion of a iireat objective, and the attention of the reader is invited to 

 the optical workshoj). Here the glass disks are first ground and 

 j)olished on both sides ]>i-eparatory to a thorough testing. Foi- this 

 purpose there is a machine with a vertical s])indle cariTing an iron 

 ])late. 



Upon this plate the glass disks are in their turn cemented with 

 pitch, and above is a sectmd iron plate, the gi'inder, j)rovided with a 

 spindle in the center. By means of this spindle the grinding plate 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 machine 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 must appear bright in the 

 polariscope, w'ith the exception of being marked by a regvdar 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 prepared from them, Avhose refractive indices are 

 determined by means of the spectrometer. Upon 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 gi-ound conca^^e, the other convex, 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 oppo- 

 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 dei)ends on the proi)er guiding of the grinding 

 tool to obtain the regular spheri(;al surfaces, the operating of the 

 machine demands great experience and care, and the work requires 

 frequent testing by the application of the spherometer. AMien finally 

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

 accurate testing of the lens is made by the Toppler '* Schlieren- 

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

 ■ The fine-ground lens is now put ui)on a lathe and centered by means 

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



