THE USE OF THE BULL'S-EYE 331 



mechanical stage or to the body, as is so often done. If it is fixed 

 to the mechanical stage, when the object is moved the light will 

 require to be readjusted, to say nothing of the probable injury to 

 the stage by the weight of the bull's-eye. If it is fixed to the 

 body the light will be displaced when the focus of the objective is 

 altered. Hence the bull's-eye should either have a weighted separate 

 stand, or be attached to the stand or holder of the lamp or other 

 illuminant. 



The optical effect of such a bull's-eye differs according to the 

 side of it turned towards the light and the condition of the rays 

 which fall upon it. The position of least spherical aberration is when 

 its convex side is turned towards parallel or towards the least diverging 

 rays ; consequently, when used by daylight, its plane side should be 

 turned towards the object, and tjie same position should be given to it 

 when it is used for procuring converging rays from a lamp, this being 

 placed four or five times farther off on one side than the object is on 

 the other. But it may also be employed for the purpose of reducing 

 the diverging rays of the lamp to parallelism, for use either with the 

 paraboloid, or with the parabolic speculum to be presently described ; 

 and the plane side is then to be turned towards the lamp, which must 

 be placed at such a distance from the bull's-eye that the rays which 

 have passed through the latter shall form an inverted image of 

 the lamp flame on the wall or a distant screen. For viewing minute 

 objects under high powers, a smaller lens may be used to obtain a 

 further concentration of the rays already brought into convergence 

 by the bull's-eye. An ingenious and effective mode of using the 

 bull's-eye for the illumination of very minute objects under higher- 

 power objectives has been devised by Mr. James Smith. The micro- 

 scope being in position for observation, the lamp should be placed 

 either in the front or at the side (as most convenient), so that its 

 flame, turned edgeways to the stage, should be at a somewhat lower 

 level, and at a distance of about three inches. The bull's-eye should 

 be placed between the stage and the lamp, with its plane surface 

 uppermost, and with its convex surface a little above the stage. 

 The light entering its convex surface near the margin turned towards 

 the lamp falls on its plane surface at an angle so oblique as to be 

 almost totally reflected towards the opposite margin of the convex 

 surface, by which it is condensed on to the object on the stage, on 

 which it should cast a sharp and brilliant wedge of light. The ad- 

 justment is best made by first placing a slip of white card on the 

 stage, and, when this is well illuminated, substituting the object 

 slide for it, making the final adjustment while the object is being 

 viewed under the microscope. No difficulty is experienced in 

 getting good results with powers of from 200 to 300 diameters, but 

 higher powers require careful manipulation and yield but doubtful 

 results. 



The second simple method of securing this illumination is to have 

 the concave mirror of the microscope capable of being used above 

 the stage, 1 so that the source of light may by its means be focussed 

 on the object. Neither of these plans will answer for other than low 



1 See Jo urn. Boy. Microsc. Soc. vol. iii. 1880, p. 398. 



