liS SIWIMAKY OF CURRENT KKSKA l,'< 'II l,s RELATING TO 



The method of working is as follows. Assuming that it is desired to 

 cheek a scale at intervals of 100 mm., the Microscopes are focused on a 

 standard scale and the interval between them is adjusted by hand to be 

 as nearly as possible 100 mm. Experience shows that this interval can 

 be adjusted correctly to within iii mm. To determine the exact 

 interval, the cross wires in the left-hand Microscope are brought into 

 coincidence with one of the marks defining the standard scale and the 

 reading of the micrometer head noted. The head is then turned till the 

 cross wires of the right-hand Microscope arc on the other mark, and 

 the reading again taken. The exact distance between the points on 

 which the Microscopes are focused is thus known. The calibration of 

 a scale may be tested by a series of observations made in a similar manner. 



If desired a micrometer eye-piece with head divided to 0*01 mm. 

 can be fitted to one of the Microscopes. It will be found that this very 

 much facilitates quick adjustment of the instrument. 



New Spencer Portable Microscope.* — This instrument, which is 

 listed as No. 60 in the maker's catalogue, is shown in tig. 3. It is 



Fig. 4. 



enclosed in a metal case (fig. 4), the two halves of which are hinged 

 together. Each half is a single thin casting of magnalium. a light alloy 

 of aluminium which is resistant to weather conditions. The wall is 

 strengthened around the edge by a narrow band of increased thickness 

 which is sufficient for holding a felt buffer to make the case dust-tight. 

 This buffer is burnished into the metal. No glue is used to fasten any 

 of the pads. The outside is finished in an imitation leather enamel, 

 which is baked on and is permanent. This, together with the rounded 



* Spencer Microscopes and Accessories, 1914, p. 54. Buffalo, New York, and 

 8°> Wigmore Street, London, W. 



