ZOOLOaY AND BOTANY, MICKOSCOPY, ETC. 139 



remarkably clear and sharp, and, though not distinctly broken up 

 into dots, gave apparent indications of a want of continuity. It 

 would be interesting (he adds) if other observers who possess large- 

 angled object-glasses, and corresponding means of illumination, would 

 give their experience in regard to the new slides of these difficult but 

 fascinating objects. 



Microscopic Test-Objects.* — Under the above title Mr. E. M. 

 Nelson replied to Canon Carr as follows : — " Having worked at these 

 objects for some years, and having also kept pace with the times in 

 objectives and apparatus, I will, in answer to Mr. Carr's request, give 

 the results of my experience: 1st, the total abolition of oblique 

 illumination if one wishes to see the true structure of an object ; 2nd, 

 object mounted dry on cover. 



I use a Powell achromatic condenser, accurately centered to the 

 optic axis. The edge of the flame of a paraffin lamp, with 1/2 in. wick, 

 exactly focused on the object, without bull's-eye or mirror. This 

 illumination, with a Powell oil 1/12, N.A. 1-43, easily resolves A. 

 pellucida, dry on cover, with direct light — i. e. without slot or stop. 



If S. gemma is examined by this means, the hemispherule theory 

 is at once exploded, and the true structure (which is far more 

 beautiful) is revealed. It is something like a most delicate skeleton 

 leaf. This, however, is very difficult for a beginner. The P. 

 formosum is, perhaps, the best one to try first. Work away at that 

 until the hemispheres, which are so easily seen, give place to a square 

 grating ! To see this, with a 1/4, JST.A. • 74, will severely test the lens 

 and the observer's manipulative skill. A coarse N. lyra and a Try- 

 hlionella punctata both have square apertures, and are very easy. 

 N.B. — If the objective is much out of correction, the square apertures 

 will blur round. The next one to try is P. angulatum. In this a 

 fracture should be distinctly seen to pass through the apertures. 

 The apertures will take a rose tint if the glass is properly corrected. 



It is manifestly absurd to test an objective by a fine diatom seen 

 with oblique light, for only a small portion of a narrow marginal 

 zone of the objective is used. The central, and by far the more 

 important, part of the glass might be stopped out. 



By the central illumination, however, the whole of the objective 

 is used ; the centre by the dioptric beam, the margin by the diffraction 

 pencils. In former days one used to hear this sort of thing said : 

 ' This 1/12 is a beautiful diatom glass.' ' This 1/10 is splendid on 

 Podura, but not good at diatom resolving.' (What a fine thing for 

 the opticians ! One had to buy two glasses, one for Podura and one 

 for diatoms.) The explanation is very simple : for Podura a glass 

 must be good in the centre, and for diatoms, with oblique light (the 

 only light used in those days), good in the marginal zone. So then 

 the 1/10, which was good for Podura, and the 1/12 for diatoms, could 

 neither of them have been thoroughly corrected from their centres to 

 their margins. I have a glass in my collection which is very fair 

 on Podura when the screw-collar is in one position, and also is a 



* Eugl. Mech., xxxviii. (1883) p. 324. 



