it;; 



VIII. — Some Remarks on a German Silver Powell Portable 

 Microscope, made in 1850. 



By A. A. C. Eliot Merlin. 



{Bead February 17,1909.) 



A FEW years ago I bad the good fortune to acquire a unique and 

 very perfect solid German silver portable Powell stand dated 1850, 

 of the type still produced by that firm. Even from a modern 

 standpoint it is a thoroughly efficient instrument. It is provided 

 with the Turrell mechanical stage, and a rack-and-pinion focusing 

 substage to which centring screws were easily added. This sub- 

 staae carries an achromatic condenser in a German silver mount of 

 precisely the pattern now manufactured and having the same 

 number and description of stops, i.e., a wheel of eleven central 

 diaphragms in addition to dark ground and oblique stops, exactly 

 as figured on page 301 of Carpenter's Microscope (Eighth Edition, 

 1901). The optical part of the condenser consists of a single front 

 hemispherical lens and two doublet back combinations, the front 

 lens of the middle doublet being concave, as in the 1857 pattern. 

 Its aperture has been measured with Abbe's apertometer and 

 proved to be 0'75 KA. The initial magnifying power is 46. 

 Used as an objective it stands a solid cone of 0*45 N.A., on the 

 minute species of the blow-fly's proboscis. It yields a solid cone 

 of fully 0-68 N.A., and with it the No. 5 Leitz of 0-74 N.A., will 

 brilliantly dot P. angulatum. It is undoubtedly superior to the 

 contemporaneous Gillett condenser which was introduced pre- 

 viously in the same year (1850) and had only 0*65 N.A. 



In addition to a micrometer eyepiece, the instrument is provided 

 with three Huyghenian oculars and six objectives. The oculars are 

 respectively of 2*0, 0*96 and 0*495 in. foci, yielding in the 10 in. 

 tube, magnifications of 5, 10 • 4 and 20 • 2. The objectives have been 

 carefully measured and tested, with the following results : — 



