176 



C. ROUSSELET ON A SMALL PORTABLE 



this purpose the concave mirror has a very short focus (l-|in.), and 

 acts as a very effective silver side-reflector. The focussing' is effected 

 by a good coarse rack-work ; a fine adjustment was not required for 

 my purpose, as I can work very well with a ^in. objective without 

 it, but it could be added if thought desirable. The stand is very 

 light, but sufficiently steady for all practical purposes. 



The whole is packed into a small case, which, when quite com- 

 plete, weighs just under 61bs. 



Fig. 7. 



I have also to bring to your notice a live box which has been 

 made to my order, which for pond life certainly works better than 

 any other contrivance of the kind I have seen. The old live box, 

 which has done duty for so long, has the very great defect that the 

 object placed thereon is totally out of reach of the substage con- 

 denser, and, therefore, incapable of being properly illuminated. 



Some years ago Mr. Swift made an improvement by fixing the 

 glass plate, on which the object is placed, nearly flush with, the plate 

 of the live box, as is shown in the above woodcut (Fig. 7). But this, 

 however, introduced another defect, that was, that any objects placed 

 in the box could be examined, over the whole field, only with low 

 powers, whilst with high powers only those objects placed near the 

 centre could be reached. Now, it is very frequently desirable to 

 examine an object witli a high power after it has been found with a 

 low one, and we all know how very fond living creatures are of 

 getting to the edge of the drop of water in which they are placed, 

 and to shift them to the centre is frequently a very tedious work, 

 and is often fatal to the animal. 



To remedy this defect, I have had a live box constructed in which 

 the glass tablet is somewhat reduced in diameter, but the outer ring 

 is enlarged sufficiently to allow any high power to focus to the very 

 edge of the glass tablet, and the result is very satisfactory. An 

 object lying anywhere in the live box can be reached by the con- 

 denser from below, and by both low and high powers from above; 

 besides which, it acts as a very good compressor, capable of fixing, 

 without hurting, the smallest rotifers, and, when you know how to 



