ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 803 



focus the mirror so as to throw light through the slide; with very 

 little manipulation of stage and mirror you will find there is a 

 position of the field in which, with 1-in. power, the centre of the 

 slide has the objects illuminated on dark ground. A very little 

 practice will eflfect this. You can now change for any object of the 

 class you wish, not moving either mirror or stage ; but you will find 

 that if you now put on, say, a 1/4-in. objective, you may have to move 

 the stage a very little to get the full effect ; you will also find that 

 by using lateral movement only you will get with the high powers at 

 the edge of the dark field a pearl-coloured light, giving most beautiful 

 definition. 



From some that I have constructed with very small lenses I have 

 been astonished to find the comparatively large field I obtain. I get 

 by the above means a result such as I had never conceived possible — 

 effects most beautiful ; good slides of P. angulatum (MoUer's) with 

 1/4 in. are lit up as with electrical light, on what I may well call 

 perfect black background, and this with wonderful definition. The 

 way all the beautiful markings of all the coarser diatoms are brought 

 out is most satisfactory. The Podura and other scales I certainly 

 never had really seen before as I can now see them. With a Zeiss 

 1/14 I get beautiful definition of everything short of A. pellucida. 



"\r\Tiat I chiefly claim for the invention is, however, not simply 

 the results thus obtained, but that they can be so obtained with 

 scarce any trouble by a simple apparatus of small cost, thus giving 

 to those who cannot afford the more or less costly affairs now in use 

 equal means of enjoying the study of this class of objects. 



I have fitted some to the substage of my large stand with 

 advantage ; but these would be more costly, as they require a 

 different position of the parts of the instrument, and are not so 

 readily applied. 



I have arrived at one fact in experimenting, which I have not the 

 scientific knowledge to explain. Say that I have some P. angulatum 

 well shown with high power, and that the background is very black ; 

 strange (to me) to say, by shutting in the binocular prism it makes 

 this ground even darker still. ... I use no condenser to throw light 

 on the mirror, only a common reading-lamp with small flame ; either 

 this, or the white cloud of daylight, answers every purpose. The 

 apparatus is constructed to work with the source of light on the left 

 hand." 



S. C. S. says * that the above " leaves microscopists no wiser than 

 they were before," and " hopes, if his lordship really wishes to benefit 

 his fellow- workers with the Microscope, he will publish his formula 

 for the construction of the Diatomescope," but this his lordship 

 objects to do.j 



Hardy's Collecting Bottle.J — Mr. J. D. Hardy devised this 

 apparatus for collecting and examining aquatic specimens whilst out 

 on excursions. It consists of two plates of glass with a narrow strip 



♦ Engl. Mech., xl. (1884) p. 18. f Ibid., p. 38. 



X Joiim. Quek. Minr. Club, ii. (1884) pp. 55-C. 



