MICROSCOPES 35 



constructed for tube-lengths of 160 or 170 mm., and only 

 to order for 250 mm. An old-fashioned instrument with 

 a fixed 10" tube should only be bought under special 

 conditions, and if of very superior make, as were those 

 of Powell and Lealand. The cheaper ones are usually 

 very liable to vibrations, and in any case the fixed tube 

 is a disadvantage for refined work. 



Diameter of Tube. The width of the tube is important, 

 as, when working without eyepieces, it governs the area 

 of the projected disc, and, within the covering power 

 of the objective, the size of the object that may be 

 photographed. Tubes are usually made 37 mm. (1J") 

 in diameter, but for photomicrography 50 mm. is much 

 to be preferred. There are several makes of intermediate 

 size. To take full advantage of the wide tube, provision 

 must be made for the removal of the draw-tube. The 

 interior of the tubes must be dead black, so as not to 

 give rise to reflections. 



Illustrations are given of several stands with which 

 the writers are familiar. They are selected to show 

 as wide a range of ' constructional details as possible, 

 and for convenience in description and comparison 

 divided into three groups, according to the complete- 

 ness of their mechanical equipment. They can all 

 be very highly recommended, but that must not be 

 taken to imply that others not described are not equally 

 good. The instruments turned out by the well-known 

 makers are of such consistently high standard that the 

 choice of a particular microscope is generally more a 

 matter of convenience, or on account of the presence of 

 some detail not found in others, than of general ex- 

 cellence or efficiency. The makers usually have stands 

 corresponding to each group, and at approximately the 

 prices mentioned below for the stand alone, without 

 optical equipment or accessories of any kind. 



Group 1. Large microscopes costing 15 and upward. 



