ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 407 



Diaphragms for Limiting the Apertures of Objectives. — Mr. 

 J. B. Dancer proposes * other forms of cliaj)hragms for this purpose, 

 the iris-diaphragm being unsuitable on account of the "ragged" 

 outline which it gives. The lirst form is an oblong plate of 

 diaphragms, which slides in an adapter screwed to the nose-piece, the 

 second is a circular rotating plate. 



The third method utili25es the ordinary double nose-piece. A 

 shallow recess is made in the top edge of one of the female screw 

 parts of the nose-piece to receive thin metallic (numbered) disks, 

 having holes of suitable diameters. A disk with the required aperture 

 can be dropped into the recess by merely moving the arm, which 

 carries the objective, on one side. A wire hook is useful for lifting 

 them out again. 



A still later device is shown in Fig. 73, and is a combination 

 of the first and third plans. An oblong 

 plate slides in an adapter, but instead Fig. 73. 



of being pierced with several apertures 

 of difierent sizes, it has two apertures 

 of equal size, into which can be dropped 

 the various diaphragm disks used with 

 the third plan. This gives great facility 

 for removing and changing the dia- 

 phragms quickly, and might, we think, be usefully adapted for taking 

 the diaphragms required for the diffraction experiments. 



It must be observed, however, that the object for which the use of 

 these diaphragms was suggested is not practically attainable. The 

 suggestion was founded on the fact that a low-angled objective has 

 greater penetrating power than a high-angled one, and it was considered 

 that by using a diaphragm at the back of the objective, thus cutting 

 down the aperture, an objective of wide aperture could be made to do 

 duty as a narrow-angled one also, so that two classes of objectives were 

 unnecessary. As Professor Abbe points out at p. 308, the plan adopted 

 in the construction of wide-angled objectives will not allow of such 

 a double use ; and it is still necessary to employ two classes of ob- 

 jectives, using those of small aperture when penetration is required. 



Correction-adjustment for Homogeneous-immersion Objectives.f 

 — Dr. G. E. Blackham discusses the reasons suggested for dispensing 

 with an adjustment to these objectives, viz. no risk of decentering, 

 the existence of a one best position in all objectives, the cost of the 

 adjustment, and the trouble of correcting. 



To these objections the following he considers to be conclusive 

 replies. 



First, if the brass-work is done with a degree of skill at all com- 

 mensurate with that necessarily expended on the glass-work of a 

 really first-class homogeneous-immersion objective, there need be no 

 fear of injurious decentering by the movements of the adjustment- 

 collar. 



Second, while it is true that the adjustment by means of varying 



* North. Microscopist, ii. (1882) pp. 89-90, 92. 

 t Proc. Amer. Sue. Micr,, 1881, pp. 61-4. 



