18 J. E. INGPEN ON PERSONAL EQUATION, 



rapid transmission of sensations to the brain. It is known that 

 the ear generally transmits impressions more quickly than the eye. 

 Differences exist in the power of observing ciliary and other move- 

 ments, and more especially in sensibility to vibrations, causing 

 more than one conflict between the advocates of the Jackson bar 

 and the Ross arm on the score of steadiness. Optical differences 

 are numerous, and may be classed under the heads of colour, focus, 

 and form. With regard to colour, much equation exists ; probably 

 scarcely any two observers see an object exactly alike in this re- 

 spect. In Admiral Smyth's " Sidereal Chromatics " the difficulty 

 of deciding upon the colours of certain well-known double stars 

 is narrated, and equation, in many cases, can be traced. The same 

 subject formed the basis of some acrimonious remarks respecting 

 " Bluish-green and greenish-blue " in the " Quarterly Journal of 

 Microscopical Science" for 1861. It is found that, even between 

 the right and left eye of the same person, a difference often exists, 

 one showing an object bluer or yellower than the other — in other 

 words, referring the colours of all objects to points a little nearer 

 the violet or red end of the spectrum. This subject formed part 

 of an interesting lecture by Professor Liebreich upon " Turner and 

 Mulready,"* in which he traced the blueness of Mulready's later 

 pictures to the existence of yellow crystalline lens, which was 

 affected differently by natural colours and pigments. The effect of 

 a "bluish haze " caused by partial opacity of the cornea or crystal- 

 line is also noticed, and its existence may account for certain 

 differences of opinion concerning the colour correction of objectives. 

 Some eyes have a greater power than others in distinguishing the 

 Fraunhofer lines at the violet end of the spectrum ; this in some 

 instances almost amounts to fluorescence. In a case of cataract, 

 in which a yellow crystalline lens had been extracted, this was par- 

 ticularly noticed. The effect of contrasts is also of importance — 

 white upon black giving the greatest amount of irradiation, while 

 to some eyes definition is clearest on a red, to others on a blue or 

 green field of view. Sir John Herschel advocated red for viewing 

 sun spots and measuring double stars ; some observers have used 

 full deep yellow with good effect. The eye is not achromatic, and 

 the objective is sometimes blamed for colour due to defective vision. 

 Slight colour-blindness also frequently exists, though unsuspected, 



* Delivered at the Eoyal Institution on the 8th of March, 1872. 



