( 1-^1 ) 



0.02 X 4.9 X J0-" = 1 X H)-'" Ei-gs. 



So the energy capable of causing iUo impression (jl" an extrenielv 

 feeble gHnuiier is, in Hie case of (>ccenlric vision, of Hie order of 

 1.10-1" ergs. This (pianlitv would iiave been found still iOO times 

 less if 1 had completely ada[)ted myself to dai'kness. By a corre- 

 sponding method Messrs. (Irmns ajid Noyons made such experiments, 

 which will be |)id)lishe(l lal(M-. 



The transformation of the relinal purple into retinal yellow nnder 

 the intluence of light is a reversible process. Hence it nnist depend 

 on a displacement of the equilibrium, which towards the end of the 

 adaptation is complete, in a direction opposed to the chemical forces 

 which are the cause of il. The small variation of thermodynamic 

 potential, brought about in the api)endi\ of the retinal nerve-cell bv 

 a qnantity of energy of the order mentioned, is sufticient to cause 

 a state of stinndation in this nerve-cell. Hence the natural nei-ve- 

 stimulus can be laken e\'er so much smallei- than the arlilicial slimulus, 

 e\'en in its most fa\()ui-able forui. 



B. Organ of smell. 



U one wishes to make an attempt at calculating the value of the 

 energy of the natural stinnibis in the case of the organ of smell, 

 this can be done iji the following mannei-. 



In llie so-called smelling-box ') (a closed s|)ace of (^4 lili'es, liaxiug 

 glass walls on all sides), let a smelling subslance be diliiled to the 

 utjnost degree at which il is still [)erce|)lible. Lei Ihis be done by 

 (•(unpletely evapoi'ating a few di'0|)s of the substance itself oi- of an 

 a(pieous solulion of il. T^et llieii a little air from Ihis space be suilled 

 in. The (piautity of air inhaled iji a single snitf is estimated by 

 \ ALKNTiN at 50 cc. whicli T believe to be cori'ecl, since \u snilïing 

 at my oHacto-meter (which is done unilaterally) 30 cc. are inhaled 

 oil the avei-age. If we now suppose that |)art of this air is directly 

 conxeyed to the olfactory fissure, in iIk^ most faxoiirable case the air 

 lliei'c present will be i-eplaced by fresh air |)ei'l"umed in tli(> mauiier 

 indicated. In this case ± 0.2 cc. of this air is in contact with the 

 olfactory mucous membi-ane on both sides of the uarroxv fissiir(\ 



We shall now indicate foi- a few substances the (|uaiitity that must 

 be present in 0.2 cc. in order to be exactly pcrceixcd by the smell. 

 This quantity is for 



1) Pliysiologie dcs Geriichs. Leipziy: ISO.j p. 34. 



