82 The President's Address. 



for the most difficult lined objects, the immersion system 

 may be found the best and most convenient, though glasses 

 specially corrected for examining such objects in an un- 

 covered state, might give a more reliable result. 



The late Richard Beck strongly advocated this mode of 

 observing Diatoms, and had a great number of them 

 mounted, so as to be viewed without covering-glass. Mr. 

 Ross has also experimented in the same direction. In such 

 observations broken valves of Diatoms are the most instruc- 

 tive in showing the real character of the marking, and the 

 most ready way of obtaining such specimens is to press the 

 moist Diatoms between two pieces of thin glass, allow them 

 to dry, and then separate the glass discs, in which fractured 

 portions of the valves will be found to adhere. 



The extreme angles given to object-glasses for the purpose 

 of disj^laying the most difficult surface markings, render 

 them comparatively useless for ordinary and more imjiortant 

 work ; and microscopists are now agreed as to the soundness of 

 the opinion enunciated some years ago by a former President 

 of the Society, Dr. Carpenter, in favour of angles of aperture 

 which are consistent with a due amount of penetration, and 

 which do not distort the appearance of objects by the false 

 perspectives which inordinate angles of aperture produce. 



If we regard immersion lenses from this point of view, we 

 shall perceive that their value must be very limited, when their 

 object is simply to produce the effect of extreme-angled 

 objectives ; but they may still have an important field of 

 utility, when applied to the highest powers, by their action 

 in increasing the working distance between the object and 

 the objective. 



Where the immersion plan cannot render some peculiar 

 and sjjecial service, it is open to the objections generally 

 made by English microscopists^, that the objective requires 

 frequent wiping, and that the employment of water is dan- 

 gerous to mounted objects, if any portion of the covering- 

 glass is cracked, or there should be any marginal crevice 

 through which the fluid can penetrate. 



In my address of last year, I brought before you the very 

 gratifying fact of the formation of the Old Change Micro- 

 scopical Society ; this Society, I am glad to say, has pro-- 

 ceeded well, and is prospering under the presidency of Mr. 

 Leaf. I have heard of the formation of similar Societies, 

 but I have not had any communication with them ; but every 

 year adds, and I hope will increasingly add, new evidence of 

 the appreciation in which microscopical science is held by 

 all classes, and particularly those interested in education. I 



