ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 425 



catch the first glimpse of light, revolve simultaneously the main tube 

 and also the Kttle knob carrying the glass disk, the object being to 

 secure as great an amount of light as possible. A little manipulation 

 of this kind ought to result in illuminating the object with a horizontal 

 (or nearly so) band of light. The next step will be by a slight move- 

 ment of the lamp, keeping its edge exactly towards the aperture, to 

 endeavour to make the band of light crossing the field as narrow as 

 possible, and the outlines of the band clear and distinct. By this time 

 the operator will have probably discovered that a slight rotation of 

 the main tube will separate the horizontal band into two parts, or, 

 as some of my pupils express it, ' two tongues.' The best position 

 is when these are made to coalesce as completely as possible. It is 

 also probable that in the attempts thus far made the image of the scale 

 has been well seen. When this occurs it should be at once focussed. 

 The next procedure is to correct the objective ; the correction obtained 

 by transmitted light will not suffiqe for the purpose in hand. It will 

 be noticed that as the glass is made to approach the correct adjust- 

 ment, the horizontal band of light will be correspondingly improved. 

 So true is this, that one might almost be governed thereby in the 

 adjustment of the object. Having got thus far along, and without any 

 serious mishap, it will be easy, by closing the shutter, to admit the 

 precise and most favourable amount of light, and also to try the effect 

 of sundry very slight changes in the position of the main tube, glass 

 disk and lamp. Very beautiful resolutions are sometimes obtained 

 by bringing the lamp within five or six inches and interposing the 

 bull's-eye condenser, flat side to the lamp, in which case the shutter 

 must be further closed. It will happen also, occasionally, that the 

 best exhibition of strise on very difficult objects, such as extremely 

 close Frustulia saxonica, is when the striae are placed at right angles 

 to the horizontal band of light." 



Pease's "Facility" Nose-piece. — This appliance (fig. 73) has 

 been devised to facilitate the rapid interchange of objectives. The 



Fig. 73. 



adapter nose-piece A screws on to the nose-piece of the Microscope by 

 the usual " Society " screw, where it may remain permanently. It is 

 provided with mechanism similar to that applied in the "self- 

 centering " chuck. By the partial rotation of the milled collar three 



