ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 833 



Penetration. — We gave at pp. 322-3 of vol. ii. and pp. 886-7 of 

 vol. iii. an account of the discussion wHch was started in America on 

 this subject, the view on one side being that penetrating power does 

 not depend upon the aperture, but is only residual spherical aberra- 

 tion — that " the amount of penetration increases with the amoimt 

 of the spherical aberration in the objective which has been left 

 uncorrected, and decreases in proportion as the corrections for spherical 

 aberration approach perfection." 



The notion that penetration arises from the defective construction 

 of an objective by the optician is, we need hardly say, wholly un- 

 tenable. 



We have delayed reprinting the remainder of the discussion until 

 after we could give the translation of Professor Abbe's paper on the 

 subject (see pp. 680-9), which has placed the question of penetration 

 on the scientific basis which it has so long needed. The view that 

 it is impossible for an objective to possess at the same time penetrating 

 power and perfect definition is seen to be equally untenable, the 

 defining power of an objective not being connected with its pene- 

 trating power, both low- and high-power, narrow-angled and wide- 

 angled objectives, if properly made, all being capable of possessing 

 the most perfect defining power. 



The following is a summary of the remainder of the discussion to 

 which we have referred, and is in reply to that printed at p. 866 of 

 vol. iii. : — 



Mr. C. M. Vorce* defines penetration in objectives as that quality 

 by which the objective is able to present the images of different 

 planes of an object in such close superposition, that the eye dis- 

 tinguishes them simultaneously, as the images of objects seen by 

 the unaided eye are perceived ; and he claims that the images pre- 

 sented to the eye by objectives having this quality of penetration 

 impress the mind at the instant of view with a true idea of the 

 bulk and substance of the object, and the arrangement and rela- 

 tion to each other of its parts. The reverse of this is true of the 

 images presented by objectives, in which the above described quality 

 has been sacrificed to the attainment of superior definition, and which 

 he calls " defining objectives," as distinguished from the others called 

 " penetrating objectives." Comparing the effect of these two qualities, 

 the author argues that the mind is more likely to obtain a correct idea 

 of the structure of an unknown object, if the images of it presented to 

 the eye by an objective, resemble in character those received by the 

 eye direct from natural objects, as is the case with penetrating objec- 

 tives, than if the mind is compelled to successively compare with each 

 other the images of separate parts and difterent planes, and labori- 

 ously trace out their relation to each other. He further contends 

 that penetration is equally as necessary in high-power objectives as 

 in low-power ones. The statement of the Rev. Mr. Dallinger, as 

 to the ^i^^-inch objective made for him by Powell and Lealand, 

 and which he used in his researches ujDon septic organisms, pub- 



* Amer. Mon. Micr. Jouin., i. (1880) pp. 170-1. 



