ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 669 



require the use of objectives of lower aperture and power, ready means 

 of applying the various accessories, and a firm stage with or without 

 mechanical adjustments, giving a considerable range of motion. 



" Now, since the introduction of the swinging substage, what do we 

 find to be the tendency of the alterations either made or suggested ? 

 In the case of Microscopes provided with mechanical adjustments to 

 the stage, a perilous attemi)t to reduce the thickness of the necessary 

 plates and a serious limiting of the traversing movements with an 

 increased complication of parts, which I can only look upon as a step 

 backwards. For many years past the alterations made in the stand 

 have all tended to a simplifying of construction and a consequent 

 increase of strength, with less chance of derangement of the adjust- 

 ments, and I cannot help thinking that we ought to be chary of making 

 an addition which leads in the opposite direction, and which, as experi- 

 ence shows, only gives a result which can be better and more readily 

 obtained by other means. 



" I have only to add that I, with many others, shall be pleased to 

 learn authoritatively whether anything has been done with the aid of 

 this addition that has not been better done without it. I have the 

 pleasure of the acquaintance of many possessors of Microscopes fitted 

 with this adjunct, and as yet, with one exception, my inquiries have 

 met with an answer unfavourable to it, the latest reply being that 

 the owner had found it so inconvenient that he had made a fixture 

 of it." 



Mr. Stephenson said he concurred generally in the views expressed 

 by Mr. Crouch. 



It must always be borne in mind — and this was a matter of fact 

 and not a matter of opinion — that objectives having apertures exceed- 

 ing the equivalent of 180° in air could only be fully utilized by some 

 immersion appliance (condenser or otherwise), and it was essential 

 that this should give an aperture at least as great as that of the 

 objective employed. A lens, or prism, must in every such case be 

 attached to the slide by some immersion fluid of a refractive index 

 equal to or exceeding the numerical aperture of the objective. Of 

 course the swinging substage might support an independent con- 

 densing lens throwing light on the lens or prism attached to the slide, 

 making an arrangement similar to the traverse lens of Tolles, which 

 was much simpler ; but, as the full aperture of every objective could 

 be utilized without a swinging substage, he preferred the ordinary 

 rigid form which was better adapted for axial illumination. 



Botterill Life-slide. — The description of this slide (Fig. 152) 

 should have accompanied that of ih% " Botterill Trough," given at 

 p. 148 of the previous volume. 



It consists of a brass slide 3 in. x 1 in., having a central opening 

 of I inch with a flange, upon which a circle of cover-glass is cemented, 

 forming the bottom of a cell. A narrow ring of vulcanite, &c., shown 

 in the figure, fits in the opening to provide (when required) depth for the 

 object to be examined, and another cover-glass is put over this. Two 

 small countersunk wells on either side communicate with the central 



Ser. 2.— Vol. I. 2 T 



