1 70 The Use of Low Towers with Beep Eye-Pieces. 



is greater than that of high ones, using the word in the sense 

 judiciously assigned to it by Dr. Carpenter, who tells us that 

 " the penetrating power of an object-glass (good definition 

 being, of course, presupposed) mainly depends upon the de- 

 gree of distinctness with which parts of the object that are a 

 little out of focus can be discerned ; this/' he adds, " will be 

 found to vary greatly in different objectives, being within cer- 

 tain limits in an inverse proportion to the extent of the angle 

 of aperture Hence, an objective of compara- 

 tively limited aperture may enable the observer to gain a view 

 of the whole of an object, the several parts of whose structure 

 lie at different distances from it, sufficiently good to afford an 

 adequate idea of the relation of those parts to each other ; whilst 

 if the same object be looked at with an objective of very wide 

 angle of aperture, which only enables what is precisely in focus 

 to be seen at all, each part can only be separately discerned,^ 

 and the mutual relations of the whole cannot be brought into 

 view." 



This passage plainly indicates the advantage of using glasses 

 with moderate angles of aperture when much penetrating power 

 is required ; but the principle upon which the advice is given 

 may be extended to a very wide range, if very low objectives, 

 which necessarily have small angles of aperture, are made to 

 give the needful magnification in combination with deep eye- 

 pieces adapted to that end. 



Microscopic pursuits always require sound definition, and 

 that being presupposed, we shall find that, according to their 

 nature, they demand penetrating power and resolving power 

 in very different degrees. Were it possible to combine both in 

 the highest degree, the most perfect glass would be obtained ; 

 but in point of fact the two stand in such relation that if either 

 is wanted in excess, the other must give way. The student of 

 diatoms, very curious in the minute and closely-approximated 

 markings which ornament the surface of certain valves, is con- 

 tent to surrender penetration in exchange for the enormous re- 

 solving power afforded by certain large-angled objectives of 

 Powell and Lealand, or Ross. But there is a class of inquiries 

 that stands at the very antipodes of these researches, and in 

 wliieli \<ry little resolving power is needed, provided consider- 

 able magnification can be obtained, accompanied by so much 

 penetration that uneven objects can have all their parts exhi- 

 bited al one view. When this is required — whether ithas been 

 witli or without reflexion upon the optical principles involved 

 — most mioroscopists have no doubt preferred using an inch 

 object -glass wii li a second or third eye-piece, to a half-inch with 

 a tower One; but tlir inosi striking advantage of lliis descrip- 

 tion may be obtained with a thrrr-i'iic/i objective worked up to 



