134- Sir David Brewster on Colour Blindness or 



curely closed by their constructors, has been allowed to remain 

 unanswered. 



On repeating the calculations given at the end of the paper 

 I have found two or three trifling typographical errors, which 

 are unworthy of notice; but there is one of another kind which 

 requires correction, since it is an error in computation and has 

 considerable effect on the final result. The value of P a being 

 24° 51' 10" and that of a' 21° 25' 48", a" must evidently be 

 3° 25' 22", instead of 2° 25' 22". The last step of the calcu- 

 lation then becomes 



Logsec2°l 25 48 =0-03111 



Log cos 21 28 50 = 9-96874 



Log cos 3 25 22 = 9-99922 



Log cos F a = 9-99907 



F« = 3°45' 



Assuming the latitude to be 30°, this corrected north polar 

 distance of a Draconis gives 26° 15' for the altitude of that 

 star at its lower culmination, and on taking the mean of Co- 

 lonel Vyse's measures of the angles of the descending passages 

 in the three principal pyramids we find the result to be 26° 13', 

 a coincidence so remarkable that there can be little doubt of 

 the correctness of the date assigned for the erection of these 

 vast structures ; and it is even more probable than the illus- 

 trious writer of the " Observations" supposed, that when first 

 built, the passages respecting which so much has been written 

 were " directly pointed at a Draconis, at its inferior culmina- 

 tion." 



I am, Gentlemen, 



Your obedient Servant, 



July 3, 1844. R. H. 



XXL Observations on Colour Blindness, or insensibility/ to 

 the impressions of certain colours. By Sir David Brewster, 

 K.H., D.C.L., V.P.R.S. Edin. 

 TN a very able and interesting memoir on Colour Blindness, 

 which I have just received from my distinguished friend 

 Prof. Wartmann of Lausanne*, I observe some mistakes re- 

 specting my opinions on this subject, which I find it necessary 

 to correct; and I do this with the more satisfaction, as they 

 are not of a controversial character, and as it will give me an 

 opportunity of gratifying the reader with an account of the 

 leading results to which he has been led. The passage in 



* A translation of this Memoir will appear in the 13th Part of Taylor's 

 Scientific Memoirs, to be published in the course of next month. 



