334 



pellucida and Navicula rhomboides, and after having had the opportunity of 

 examining them he was inclined to think that they marked a new era in 

 diatom mounting : the results far surpassed all that had been done in phos- 

 phorus. If the new process proved to be a success as to its permanence, no 

 doubt the details would be published in due course. 



Mr. Ingpen said they could only look forward with some eagerness to the 

 publication of a matter so likely to be advantageous to microscopists 

 generally. He thought better of phosphorus, however, than Mr. Nelson 

 did, and he had hoped to be able that evening to show some objects 

 mounted in phosphorus of a higher refractive index than had hitherto 

 been obtained. The fault with most of the slides yet exhibited was that 

 the solution of phosphorus was not sufficiently saturated ; but a friend of his 

 had been turning his attention to the matter, and was trying some experi- 

 ments with the idea of being able to mount some diatoms in absolutely solid 

 phosphorus, from which very much better results were anticipated. Until 

 his friend was able to come and state what he had done it would, perhaps, 

 not be quite safe to say much about it, but he could not think that any sub- 

 stance could have much value for this purpose unless it had a refractive 

 index as high as phosphorus. Some of the diatoms themselves possessed 

 so high a refractive index that a very little difference, or only a slight lower- 

 ing of the refractive index of the medium, would make an enormous differ- 

 ence in the visibility of the object. It was easy to see, by pressing the cover 

 glass, that in most of these slides the centre was still fluid, and that there- 

 fore there was some of the bisulphide of carbon remaining. 



Mr. Nelson said with regard to the relation of aperture to power, a question 

 which seemed to be exciting some degree of interest just now, he had made 

 an ideal table of what the various powers ought to be if they were to be perfect, 

 lin. being 30°, ^in. should be 1*56, which had yet to be made. 



Mr. A. D. Michael exhibited, and described, a specimen of the so-called 

 " lungs " of a spider ( Tegenaria doniestica) ; and illustrated his description 

 by means of black-board drawings. Mr. Michael said that this was possibly 

 the most profound of the many modifications to which trachea? were subject. 

 The primitive tracheal system was, probably, that paired, lateral stigmata 

 existed in the intersegmental membrane between each somite ; and that, from 

 each of such stigmata, a single unbranched trachea proceeded inward to 

 supply the somite. Something like this existed in some Myriapoda, and was 

 very instructive. In the Acarina the somatic arrangement was lost, not 

 only in the tracheae, but also in the whole structure ; yet the simple, un- 

 branched condition of the tracheae was often preserved, as in the Oribatidce ; 

 but the stigmata were few, and usually near together. In the Insecta the 

 tracheae anastomosed so freely that the simple type was quite lost. We got 

 the intersomatic stigmata ; but the tracheae, after proceeding inwards for a 

 short distance, usually united so as to form (as in the larvae of Lepidoptera) 

 a great lateral trunk along each side of the body, from which numerous, 

 richly-branching trachea? arose, distributed in profusion to the various 

 organs. In spiders two classes were found, one with only tracheae of the 



