5^6 



NATURE 



[September 30, 1897 



oxygen gas of atmospheric density, at the temperature 

 of freezing water, is exactly 1/700 of the gravity of 

 distilled water, at its temperature of greatest density. 

 He advocates this as an international circle of latitude 

 for all gravitational calculations. The author says that 

 "from this fact as a starting point, all fundamental 

 values have been determined, and expressed with 

 absolute exactness in units and vulgar fractions instead 

 of approximately by rows of decimals," and he claims 

 that his arithmetical method gives an "absolute accuracy 

 of results, and a facility of manipulation not attainable 

 by any other known method." 



It is possible that the use of convenient vulgar 

 fractions for physical constants may conduce to facility 

 of arithmetical manipulation, but the author, for the 

 sake of his vulgar fractions, makes assumptions which 

 can surely not conduce to the absolute accuracy which 

 he claims. For example, he takes 17/12 as the ratio 

 between the specific heats of gases at constant pressure 

 and at constant volume, because 17/12 is a simple 

 fraction not far removed from the determined value of 

 the ratio for simple gases, and, moreover, in spite of 

 experimental evidence to the contrary he uses the same 

 ratio in the case of such gases as CO2. The author 

 also advocates, and uses, a new scale of temperatures, 

 not very different from the absolute Centigrade scale, to 

 facilitate his arithmetical work. This he calls the 

 normal scale. 



The author makes a great number of calculations, 

 arranging the results in tables. Where experimental 

 evidence is at variance with any of these results he con- 

 siders the experiments are inaccurate. The arithmetical 

 work is conducted with considerable ingenuity, though 

 occasionally the mode of statement of details is not un- 

 exceptionable, e.g. (on p. 24), log o'ooooo - log o"2i249 

 = log 178751 = o'6i307. 



Prof G. Karsten, of Kiel, has written an introduction 

 to the book, in which he calls special attention to the 

 author's proposal (mentioned above) that all observations 

 and calculations on gravity should be referred to one 

 common international circle of latitude, to be called the 

 circle of international gravity. He also mentions § 80 

 and Table xxiii. as samples of the satisfactory results of 

 the author's calculations and observations on heat pro- 

 duced by combustion, and recommends the book to the j 

 attention of scientific men. j 



The book on the whole, though the calculations are, in 

 many parts, of considerable and varied interest, does not 

 seem to justify its ambitious title. 



Citizen Bird: Scenes from Bird-life i7i plain Ene,lish 

 for Beginners. By Mabel Osgood Wright and Elliot 

 Coues. Pp. xiv -i- 430. (New York : The Macmillan 

 Company. London : Macmillan and Co., Ltd., 1897.) 

 This book consists of a series of pleasant dialogues 

 between Dr. Roy Hunter and some children, at Orchard 

 Farm in New England, in which the children learn the 

 appearance and habits of a great number of the birds ! 

 around them. It has been rather unfairly compared in i 

 a daily paper to " Sandford and Merton." It must be I 

 allowed that the didactic dialogue is apt to be tiresome, 

 and in this case the children are of course a little un- 

 natural m their acuteness and their ardent desire to 

 learn. English boys would probably learn better from 

 a sound and scholarly handbook : one in whose hands 

 I to-day placed Sir Humphry Davy's " Salmonia," after 

 a few days' trout-fishing, not unjustly complained that 

 Halieutes and his pupils always caught exactly the fish I 

 they wanted — which was not the case when he was I 

 fishing. It may perhaps be doubted whether the i 

 experiment would answer on this side the water. j 



But the familiar names of Dr. Coues and Miss M. A. 

 Wright are a more than sufficient guarantee of the 

 excellence of the ornithological part of the book, and to 



NO. 1457, VOL. 56] 



I English students of bird-life it will be of real value. 



I Here we have the actual every-day life of the birds most 



familiar to the New Englander,. which very few of us 



j can hope ever to study in their own homes. Many 



I of them, of course, closely resemble our own, and a 



I very few are identical with ours. But the great majority 



are new to us, land of these we learn very pleasantly 



from this book something that we could not have picked 



up except by crossing the Atlantic ourselves. The 



photographic illustrations are excellent ; and there is 



a useful index and a classification of North American 



birds. But perhaps the best thing in the book is the 



account given by Mammy Bun, the negress, of the 



mocking-bird as she knew it in the Southern States. 



W. Warde Fowler. 



LETTER TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to retu7-n, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. ] 

 The Worsted Test for Colour Vision. 

 L\ Nature of September 23 reference is made to the death 

 of Dr. A. F. Holmgren, professor of physiology in Upsala 

 University. "His attention," it is said, "was in the early 

 seventies directed to colour-blindness, and in 1878, he published 

 his well-known work on colour-blindness in relation to railways 

 and the Navy, thus bringing to a practical issue the work long 

 before begun by George Wilson, of Edinburgh (1855). This 

 led him to the invention of his now well-known worsted test for 

 colour vision." 



May I be allowed to say that Prof. George Wilson, of Edin- 

 burgh (my brother) was, during a long series of investigations as 

 to the nature and extent of thispeculiarity of vision, constantly in 

 the habit of using the " worsted test." In his work, " Researches 

 on Colour Blindness" (published in 1855 by Messrs. Sutherland 

 and Knox, Edinburgh, and Simpkin, Marshall, and Co., London), 

 references very frequently occur to the use of wools as a colour 

 test. On page 25 he says, " Dr. Y. , aged 27, when requested to 

 match coloured worsteds by daylight placed the full reds and 

 greens together, but when the same skeins were placed before 

 him by gaslight, he picked out the greens and placed them 

 apart," 



At page 44, while examining artillery soldiers at Leith Fort, 

 he put into the hands of one man a bundle of coloured wools, 

 from which he was to make a selection. The soldier was 

 nervous, but retained with firm grasp a yellow skein of wool, 

 putting it in the bundle containing red purple and red 

 brown, with manifest perplexity at all the colours being alike. 

 Page 40, soldiers in the Edinburgh garrison, known by previous 

 experiments to Ije colour-blind, were closely watched while 

 from a heap of coloured wools each one was asked to select first 

 the red skeins and then the green, no notice being taken of the 

 selection made till eight or nine skeins were set aside as red and 

 the same number as green. 



At page 70, 437 soldiers were asked to assort coloured papers, 

 wools, and pieces of glass, and to place those of the same hue 

 together. At page 77 a young Kafifli gentleman, whose know- 

 ledge of English was limited, was asked to match Berlin wools 

 and tinted papers. 



One advantage gained by making wools the test, was that 

 many of the colour-blind have a specially keen sense of minute 

 details, so that in seeing the same object more than once, they 

 would recognise it by some small point or wrinkle or crease, 

 scarcely perceptible to an ordinary observer. In the wool test 

 this power was of no service to them. 



I think from the examples quoted (and many more might be 

 given), priority in making use of the "worsted test" may 

 fairly be claimed for Prof. George Wilson. At many of the 

 lectures given by him on this subject a diagram was exhibited, 

 consisting of a square of calico to which were attached speci- 

 mens of wools as selected by the colour-blind tested by him. In 

 the course of time the colours faded, so as to lessen its value, and 

 it was put aside. Jessie A. Sime. 



12 AthoU Place, Edinburgh. 



