524 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



SO as to como nearly in contact with the glass surface. It would, 

 wc think, be preferable that the ebonite diaphragms should be flat so 

 as to b) applied to either side of the mirror. 



The suggestion of applying diaphragms to the mirror is a revival 



of a plan adopted towards the end of 

 the last century by Dellebarrc. He 

 made disks of thin metal with circular 

 apertures of ditlerent sizes to be 

 ai)plied as caps over the plane or 

 concave surface of the mirror, being 

 held in position by three angle-pins 

 projecting from the jjeriphery. 



Centering the Illuminating 

 Beam.* — Mr. J. W. Queen points out 

 that it is a practical point of much 

 importance in the use of the Micro- 

 scoi)e to have strictly central light. 

 To secure this he is accustomed to 

 pursue the following plan : — 

 Having the object in place and lighted from the mirror, the 

 objective screwed on, and the eye-piece in the tube, first focus. Then 

 remove the eye-piece, and applying the eye centrally to end of the 

 body-tube, notice the spot of light at the back of the objective. It may 

 appear as in A, in which case move the mirror or diaphragm, or both, 

 until the illuminating beam ajipears central, as in B. If now the lens 

 is a good one, properly adjusted, and the inner circle of B jircsents 

 an evenly illuminated disk, a good, sharply defined image should be 

 obtained. But there may not be light enough, or there may be too 

 much, in which case a diai)hragm of different size should be used, or 

 its distance from the object varied, thus varying the angular size of 

 the illuminating pencil. If the diaphragm be too large or placed too 

 near the object, it ceases to afiect the angular size of the illuminating 



Fig. 115. 



beam (although it may act in another way), and in this case the image 

 of the mirror is seen within the circle of the diaphragm, as at C, if 

 the objective is of sufficient aperture. 



If lamplight be used, the image of the flame may be seen within 

 the disk of mirror and diaphragm, as at D. This shows that the beam 

 is not focused upon the object. This may be remedied by the use of 

 a condenser placed near the source of light, making parallel the rays 

 falling on the mirror, or simply by altering the distance of the mirror 



Micr. Bulktin, ii. (18S5) p. 1 (4 figs.). 



