562 SUMMARY OF CUREENT RESEARCHES RELATING TO 



this value becomes with the new micrometer the easy quantity 2 [x. 

 This value multiplied by the number of divisions covered (e.g. 17) 

 would give the length of the object (i.e. 34 /jl), a result easily attained. 

 Similarly the micrometer values for Leitz objectives 1, 2, 3, 4, 

 are respectively 30 /a, 15 fx, 10 /*, 5 ft, provided that the proper 

 tube-lengths are used. Some of the micrometer values are still 

 simpler, no fewer than five being unity itself. The author gives 

 a full table of statistics referring to Leitz objectives. The fourth 

 column of the author's table shows how many divisions of the object 

 micrometer must be covered by 100 divisions of the step-micrometer, 

 and, in the case of other objectives, the necessary tube-length must 

 be determined experimentally once for all. As an illustration the author 

 quotes the observation of a valve of SurireUa gemma by objective No. 6 

 and by oil-immersion ^^. Measurements were made (1) by an ordinary 

 travelling micrometer ; (2) by a step-micrometer. The four results are 

 practically the same (107 • 4 /x to 107 • 8 /x), but the step-micrometer has 

 an undeniable simplicity in calculation. For the travelling micrometer 

 the actual figures are: (objective 6) 30-9 X 0-00349 = 0*1078 mm. 

 = 107-8ft; and (^V) 65*5 X 0-00164 = 0-1074mm. = 107-4 fi. 

 For the step-micrometer : (objective 6) 53-8 x 2 /a = 107-6 ft; and 

 (tV) 107-5 X 1 ft = 107 -5 ft. 



(4) Photomicrograpliy. 



Blue Screen.* — E. W. Bowell gives the following directions : — Take 

 an unexposed process plate, fix it out in the dark with " hypo," wash 

 well. Fix the gelatin film with formalin (say 10 p.c), as if it were a 

 histological preparation ; this may take several hours. Wash well, and 

 stain with aqueous solution of acid-violet (saure-violett Griibler) ; wash 

 and dry. This screen will transmit only blue and half the green. The 

 staining was made by adding water to a saturated solution of the dye- 

 stuff in 70 p.c. alcohol. 



Duplex Photo-micrographic Camera. f — This camera (fig. 101), by 

 W. Watson and Sons, has been designed for use either vertically or 

 horizontally, and to obviate difficulties with Microscopes of different 

 heights and centres ; the bellows are arranged for raising and lowering 

 to suit the various axes. It is constructed principally of metal, so that 

 it may be suital^le for tropical climates. 



Watson's "Laboratory" Camera. | — This camera, by W. Watson 

 and Sons, retains the heavy mahogany base, but has been re-modelled 

 in certain other respects, particularly the bed and guides which carry the 

 camera body itself ; these are now made in metal, and the whole instru- 



* Knowledge, xxv. (1912) p. 343. 



t Photo-]\Iicroprojection Catalogue, p. 10. 



J Photo-Microprojection Catalogue, p. 6. 



