52 Dr. W. Pole on Colour-Blindness* 



that indicated by the law for the power developed. If the 

 average weight were much less the discrepancy would be 

 lessened. But I think I remember seeing Leslie Stephen 

 described somewhere as " the fleetest-footed of all the Alpine 

 brotherhood ; " so that probably his party might be expected 

 to show an endurance in excess of the average, at a high 

 speed of ascent. 



The bearing of these considerations on the use of such an 

 expression as " horse-power " is important. However con- 

 venient it maybe as a short name for 550 ft.lbs. per sec, it is 

 clear that no specification of ft.lbs. per sec. can really repre- 

 sent the work of any animal, unless the endurance, or the 

 time for which that power can be maintained, is also specified, 

 and the law connecting the two is known. 



V. Further Data on Colour-blindness. — No. III. 

 By Dr. William Pole, F.R.S.* 



Professor von Helmholtz's Handbook of Physiological 

 Optics. 



OF all the names that could be mentioned as authorities 

 on the subject of colour-blindness, probably that of 

 von Helmholtz stands the highest, not only from the asso- 

 ciation of his name with that of Young in the most popular 

 theory of colour- vision, but more especially on account of his 

 monumental Handbuch der Physiologischen Optik, which has 

 acquired a classical celebrity. This work appeared at inter- 

 vals from 1856 to 1866 ; but scientific knowledge advances 

 much in a quarter of a century, and the learned author is 

 conferring a great boon on the public by giving them a new 

 edition, thoroughly revised and brought up to date. To 

 make it more accessible it is published in separate a Liefer- 

 ungen," of which seven have appeared, comprising 560 

 closely printed large octavo pages. 



I have been surprised to find how little this new edition is 

 known in England ; and as the part treating of colour- 

 blindness is already out, it would be culpable, in offering 

 data on this subject, to omit reference to it, particularly as 

 one of the special objects of this Magazine has always been 

 to call the attention of the English-reading scientific world 

 to important foreign publications. The subject mentioned 

 forms, indeed, but a minute item in the whole treatise ; but 



* Communicated by the Author. 



