LIMITS OF OPTICAL CAPACITY OF THE MICROSCOPE. 431 



Were it possible to obtain with an immersion lens, the transmission 

 of rays = 180** of divergence aperture (in water) a would then 

 = 90° and \ nearly f X^ 



and hence e = ^^ mm. =(l^o ^°''^) 



According to measurements of Harting (published in vol. 114 of 

 Poggendorf 's annals), the magnitude of the smallest distances taken 

 with J^o 10 objective of Hartnack, reckoned by our formula is 



1 

 £ = mm. 



3,313 



The figures ^-^^ o^ d^^i. given by Harting refer to the width of the 

 dark space between the lines. In close accordance with the above 

 are the measurements by Herr L. Pippel (in his work on the 

 microscope, Brunswick, 1867), of fine diatoms, who found that the 

 closest series of lines that he could distinguish =-3-3-^^ mm., and 

 the finer Nobert lines = 3-5^0 (uoooo ii^ch). Earlier measurements 

 1853, of Messrs. Sollitt and Harrison (Quarterly Journal 

 Microscopical Society, vol. 5, p. 62) count much higher. Eecog- 

 nisable lines Navicula Arcus are said to have been counted at 5120 

 to the mm. (tWooo i^ch). This far exceeds the theoretical limits 

 for objects in air. But since all later measurements remain much 

 lower than these, I do not know that they are trustworthy. Harting, 

 also, who cites them doubts their accuracy. 



Besides, any possible further increase of angular aperture in the 

 case of objects lying in water, the capacity of performance might, 

 perhaps, be increased by employing blue rays only.* 



In photography, blue light is chiefly active, and photographs 

 appear actually to perform more than the eye can with white light. 

 In a photograph of Surirella gemma, executed by Dr. Stindi, with 

 an objective of Gundlach's, giving LQim amplification, L'nes are 



* Hartnack makes an illuminating apparatus for use of blue rays only, and 

 exhibited it in the Vienna Exhibition, 1874. 



