ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 229 



a drop of water hanging upon the surface of a glass plate, can be ren- 

 dered conspicuously visible by cutting down the image-forming beam in 

 a particular position. The experimental result goes somewhat beyond 

 the theoretical explanation at present available. The experiment itself 

 had been designed with the idea of introducing a diffraction fringe into 

 the field of the instrument and utilising the high resolving power which 

 such a fringe is, on grounds of theory, supposed to possess. But the 

 increased visibility of the image shown up in this way is so very pro- 

 nounced that there is reason for thinking that the diffraction fringe 

 does not completely explain it. The result is to exhibit a transparent 

 object under the aspect known to microscopists as dark field illumina- 

 tion, but without any of the appliances commonly employed for bringing 

 about that result.* 



Another appliance directed to the same general object of improving 

 the resolving power of the Microscope is the apparatus devised by Dr. A. 

 Kohler for utilising ultra-violet light having a wave-length of about 

 ^^nr in. A description of this apparatus has already appeared in this 

 Journal, 1905, pp 103 and 513. The exclusion of glass from the optical 

 system makes it impossible to obtain the ordinary corrections for chro- 

 matic aberration, and the objectives prepared for use with this instrument 

 are accordingly designed as monochromatic, that is to say, they are correct 

 for one particular wave-length only. This elaborate instrument is at 

 present in the probationary stage, and it does not seem possible to speak 

 as yet with confidence of its capacity for high-power work. But its use 

 at moderate magnifying powers has shown that light of the particular 

 wave-length mentioned is arrested by certain tissues which are quite 

 transparent to ordinary visible light, and in that way structure can be 

 demonstrated in unstained specimens by the aid of colour reactions by 

 latent stains, as we may say, to reveal which ordinary light would be 

 wholly inoperative. Special attention was drawn by the lecturer to this 

 very notable property of monochromatic light and to the simplification 

 of the correction problem accomplished by the production of mono- 

 chromatic objectives. In both these particulars he thought that the 

 principles exemplified by Dr. Kohler's design might usefully be extended 

 to instruments designed for working with monochromatic light within 

 the range of the visible spectrum. 



The last subject dealt with was a photomicrograph ic apparatus 

 recently introduced by Messrs. Beck. This is designed to meet the 

 requirements of practising engineers and doctors, and reduces the appa- 

 ratus and procedure employed in the production of a photograph of a 

 microscopic object to extreme simplicity. 



A full description was given at a recent meeting of the Society, and 

 may be found in the Journal, 1905, p. 651. 



Quekett Microscopical Club. — At the Meeting held on Jan. 19, 

 the President, Dr. E. J. Spitta, F.R.A.S., F.R.M.S., in the chair, the 

 Hon. Sec. announced that two papers had been communicated by Mr. 

 T. B. Rosseter, F.R.M.S., on " Drepanidotcznia undulata," and on 

 " Drepanidotamia sagata." 



* This experiment forms the subject of a Note on p. 157 of this Journal. 



