ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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(b) By having two auto-collimating telescopes, not only is a check 

 obtained by taking an observation through one eye-piece and then 

 through the other, but a further check is readily obtained by passing the 

 light through the prism in a similar but opposite manner. 



(c) The error due to auto-collimation is eliminated when method (1) 

 (normal incidence) is employed, by taking the mean of two observa- 

 tions — with illuminating prism and cross-wires to right and left, or 

 vice versa, of field respectively. The same applies to the "critical 

 incidence " method, which, however, is not apt to produce such accurate 

 results, owing to the difficulty met with in placing the cross-wires to 

 exactly cut the half-light and half-dark field. 



Optical Bench for Illumination with either Ordinary or Mono- 

 chromatic Light. — The firm of R. and J. Beck has designed a small 

 optical bench for microscopical illumination, which is suited for use 

 with the Rosenhain Microscope, as well as for other purposes, and has a 

 series of very handy adjustments (fig. 32). 



Pig. 32. 



It consists of a 30-in. steel bar mounted on two strong tripod feet 

 loy means of two sliding uprights which can be raised or lowered and 

 clamped at different heights, in fittings carried in the tripod feet, so 

 that the bar can be placed at different heights above the table, or can 

 be pointed either upwards or downwards. 



All the pieces of the apparatus slide along the steel bar, and are 

 accurately aligned to the same centre or axis. Each piece can be clamped 

 in any position. 



At one end A is a Nernst lamp with a single upright filament 

 giving from 100-230 candle-power. It can be used with a slight 

 modification in the burner on any current from 110-220 volts, but to 

 give the highest candle-power should be used with the highest current. 

 Next to the lamp is an achromatic and aplanatic condenser B, If in. 

 diameter, which is corrected to give a well-defined image of the filament 

 on the slit C ; this is adjustable by means of a screw. 



An achromatic collimating lens D collects the light from the slit C 

 and emits it in a parallel bundle to a Thorp's diffraction grating 

 on a prism, with rotating adjustment and brass cover, which transmits a 



April 18th, 1906 q 



