7G G. WILLIAMS ON THE USE OF 



platform beneath its springs, the flat side of the "bull's-eye" is 

 brought close up to the side edge of the slip, a brilliant beam is 

 concentrated upon, and thrown across, the slide, the proper illumina- 

 tion is obtained by the necessary manipulation, and the markings are 

 thus resolved. 



In order, however, to show these transverse lines or striae with 

 this "bull's-eye," it is necessary that they lie at right angles to the 

 direction of the light. A selected slide, prepared on purpose, of 

 course provides for this, the mounter arranging the frustules on the 

 cover, and fixing it so that they lie across the slip ; but, to say 

 nothing of the great difficulty of thus arranging A.pellucida (which 

 high skill and much patience can alone accomplish), the number of 

 the valves is generally limited to some half-dozen, and these few 

 mostly turn out to be so extremely difficult of resolution, and are 

 otherwise so unsatisfactory as to render the slide of comparatively 

 little use. A good ordinary strewed slide, on the contrary, contains 

 hundreds of the valves, and amongst them may be found, by diligence, 

 many on which the lines can be seen with reasonable ease ; but it is 

 obvious that the "bull's-eye" can be made to successfully operate 

 upon those specimens only which happen to lie in the direction of 

 the width of the slip, or within very narrow limits thereof ; the rest 

 of the specimens, and possibly the best, being incapable even of 

 examination owing to their unsuitable position. To place any one 

 of the latter in the right direction it is clear that the only course is 

 to turn the slide by the rotating motion of the microscope ; but the 

 length of the slip quickly interferes with, and pushes aside, the 

 " bull's-eye," and there is nothing for it but to abandon the examina- 

 tion of that valve. To overcome this difficulty, and put an end to 

 the annoyance of seeing crowds of frustules which one cannot even 

 attempt to resolve, I have devised the easily-made and inexpensive 

 apparatus described below ; and I have ventured to bring it to the 

 notice of the members of the club, in the hope that it may meet 

 with approbation, and extend the use of this excellent illuminator, 

 which will be found to give very good results with A. pellucida, and 

 to be equally effective in resolving other tests ; always, of course, 

 assuming that an objective be employed which is competent to the 

 task. 



Fig. 1 is a disc of thin plate, or crown, glass, f " diameter, on 

 which may be mounted diatoms in balsam, or dry strewed on the 

 cover. Any cover carrying dry diatoms can be easily removed from 



