NELSON'S COMPUTATION 333 



Glass, boro-silicate, the same as before. 



Radii r = -f 272) ,. 



diameter 2-1; 



diameter 1-9. 



Distance of lenses apart '05, equivalent focus 2'0, working dis- 

 tance 1-50, angle 60. 



It is illustrated in a mounted form in fig. 278. Combinations 

 having different foci may be constructed in the same manner as in 

 the example above. 



An illuminator not so well known, or at least so much used, as 

 its merits justified, is Powell and Lea*land's small bull's-eye of J inch 

 focus, which slides into an adapter fixed into the sub-stage, and 

 susceptible of its rack motion upland down. The object is placed 

 on a super-stage, and lies considerably above, but parallel with, the 

 ordinary stage. The bull's-eye, capable thus of 

 being raised or lowered, and of being moved by 

 sliding away from or close to the mounted object, 

 has its plane side placed against the edge, and at 

 right angles to the plane of the slip. By this 

 means illumination of great obliquity can be 

 obtained, and very surprising effects secured even 

 with high powers. It was much used by the Editor 

 and Dr. Drysdale in their earlier work on the 

 saprophytic organisms, and, in the days before 

 homogeneous lenses, helped us over many diffi- 

 culties of detail. It was the first illuminator to 

 actually resolve the Amphipleura pellucida. It 

 could be very easily obtained with a student's FIG. 278. Bull's-ey 

 microscope provided with Nelson's open stage, 1 for of good but not the 

 on this the bull's-eye could be placed against the 



, , ,. ~ , L . . 



edge of the slip without any special apparatus or 

 fitting. 



Another and popular method of ' opaque illumination ' is by 

 means of a specialised form of mirror, generally of polished silver, 

 called a side reflector, and fixed, as in the case of the bull's-eye, and 

 for the same reasons, to an immovable part of the microscope. 



The manner of employing this reflector, as provided with Powell 

 and Lealand's best stand, is seen in Plate III. The arm of the 

 side reflector is fixed to an immovable part of the stand, and is thus 

 unaffected by the racking up or down of the body. The lamp placed 

 on the right of the observer is set at such a height that its beams 

 fall full upon the reflector; this, by means of a ball-and-socket 

 joint, can be easily manipulated until the full image of the flame is 

 caused to fall upon the object. For the same purpose a parabolic 

 speculum is commonly employed, mounted either on the objective, 

 as in Beck's form, fig. 279, or on an adapter, as in Crouch's, shown 

 in fig. 280, where a collar is interposed between the lower end of 

 the body of the microscope and the objective seen at A. This is not 



1 Fig. 134. 



