360 SUMMARY OF CURRENT RESEARCHES RELATING TO 



rests on the point of the micrometer screw, by means of which it is 

 actuated. The screw has a pitch of 1 mm., and its head is divided into 

 10 parts. 



The telescope rests in V's, and is reversible. It has a Kamsden 

 eye-piece, adjustable diaphragm with cross wires, and a movable object- 

 glass for focussing. Two spirit levels are supplied on the base and one 

 on the telescope. 



An Old Rackwork Draw-tube. — Mr. Nelson writes to say that 

 he was shown an old monocular Microscope fitted with a rackwork 

 draw-tube ; the tube was graduated in inches and tenths. The milled 

 head on the pinion, which was geared to the rack, was divided into five 

 or six equal portions by small countersunk holes, into which a spring- 

 catch pressed. When the milled head was revolved, so that the spring- 

 catch passed out of one countersunk hole into the next, the draw-tube 

 was moved exactly one-tonth of an inch. Therefore by feeling or hear- 

 ing the spring click the amount of movement given to the draw-tube 

 would be known, without the necessity of removing the eye from the 

 eye-piece for the purpose of reading the graduated scale. 



The legend engraved on the Microscope was " M. Pillischer, 398 

 Oxford Street, London. 167." Its date is 1847-48. 



(2) Eye-pieces and Objectives. 



Beck-Steinheil Orthostigmats.— These lenses (fig. 75) were primarily 

 introduced for photography pure and simple, but owing to their excep- 

 tional qualities as to their corrections both for colour values (severely 

 tested in connection with the photo-mechanical three-colour work) and 



also for spherical and astigmatic errors, Messrs. 

 ibii= w3t«mI Beck have introduced a number of shorter foci 

 i ' 'T ii^af P lenses specially for the most difficult photo- 

 mi crographical research. Each surface is 

 l^ fJII polished and figured on the principle adopted 

 - — ,i for the manufacture of large astronomical tele- 



scope objectives, and the accuracy of the test 

 Fig. 75. employed is such that a surface error not ex- 



ceeding a fraction of a wave-length may be 

 detected. No mechanical measuring machine has ever been constructed 

 which will measure the errors of curvature with a tithe of the accuracy 

 of the method adopted. The series at present includes three members, 

 whose focal lengths are respectively 1, 2, 3£ in. ; their corresponding 

 apertures being ^, ^, T 9 ^- in. 



Leitz' Spherically, Chromatically, and Astigmatically corrected 

 Objective.* — E. Leitz, of Wetzlar, has taken out a German patent f for 

 the above, and the spherical and astigmatic corrections are accomplished 

 by a pair of reversed combinations separated from one another by a 

 film of air (fig. 76). The outer members form a biconvex lens A, and 

 the inner consist of a pair of plano-concave lenses B, and two positive 

 menisci C. The biconvex A is made out of a highly refractive crown 

 glass and serves for the rectification of the astigmatism. The spherical 



£ * Deutsche Mechaniker-Zeitung, No. 2 (Jnu. 1902) p. 19 (1 fig.). 



! f No. 118433, jCI. 42, 16.7.1899. 



