292 



NA TURE 



January 24, 1895 



in the first chapter, a concise account is given of the 

 means of naming and identifying stars, suthcient to make 

 possible an intelligent observation of the diurnal motion 

 of the celestial sphere : and in the next chapter, it is 

 shown that the observations can be explained by regard- 

 ing the earth as a spinning globe. The same method is 

 followed throughout. ■ ,- • i 



On the whole, the subject-matter has been judiciously 

 selected, but a slight want of proportion is shown in 

 introducing explanations of the phases of Mercury and 

 Venus, while those of the moon are not referred to at alK 

 The chapter on the determination of the size of the earth 

 would have been a little more educational if the descrip- 

 tion of the methods employed had been accompanied by 

 hints as to the amount of playground surveying which is 

 possible by the use of a protractor and foot-rule. 



The book forms an admirable introduction to astro- 

 nomy, which stands a fair chance of fulfilling the author s 

 hope "that this little book will help to revive the obser- 

 vational astronomv of pre-telescopic times. This branch 

 of astronomical knowledge is certainly not without danger 

 of being neglected in favour of the fascinating and rapidly- 

 advancin- studv of the results obtained by the use of the 

 camera and spectroscope. The explanations are models 

 of clearness and accuracy, and the diagrams illustrating 

 them are excellent. Many of them are new, and involve 

 original ideas of the author ; as, for instance, a diagram 

 illustrating the sun's apparent path in winter and sum- 

 mer, and another showing the principle of Foucault s 

 pendulum by a lecture experiment. Teachers of 

 geography and physiography will do well to make 

 themselves familiar with Mr. Gregory's methods. 



LETTERS TO THE EDITOR. 



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

 pressed by his correspondents. Ntither can he undertake 

 to return, 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'Hodgkins Prizes. 



The time for the reception of l realises or essays offered in 

 competition for the Hocigkins Fund Prizes of 10,000 Hols, of 

 2000 dols., and of 1000 dol«. respectively, clo-ed on Occcmber 

 31, 1894, and all papers so offered are now in the hands of the 

 Commiitee of Award. 



In view of the very large numher of competitors, of the 

 delay which will be necessarily caused by the intended careful 

 exaroinaiim, and of the further lime which may be required to 

 consult a European Advisory Committee, if one be appointed, 

 it is announced that authors are now at liberty to publish these 

 treatises or essays without prejudice to their interest as com- 

 peti'ors. . S. P. Lanclev. 



Washington, January 10. 



The Artificial Spectrum Top. 

 As the spectrum top is exciting a good deal of interest at the 

 present m .ment. perhaps I may be allowed to record some 

 experiments which I have made with a view of arriving at a 

 solution of the colour problem which it sets. I have ol)scrvcd 

 the colours produced by the white light o\ the positive pole of 

 the electric arc, and also by monochromatic lijiht produced by 

 m»ans of my colour-patch appar.itus. The top was rotate! on 

 a horiitonlal axis at any desired speed by mean* of an clectrn- 

 m'.tor. The following colours were observed (No. I, No. 2, 

 No. 3, and No. 4 are the triple lines in order from the centre 



of rotation) : 



White light. 



No. I. Crimson. 

 No. 2. Olive green. 

 No. 3. Grey ('lightly violet). 

 No. 4. Dark violet. 

 (When the yellow light of gas is used, the above results would 

 be m'idificd) 



Ked(Clishl). 

 No. I. Red. 

 No. 2. Lighter red. 

 No. 3. Very light olive green. 

 No. 4. Darker olive green. 



\ 



Green (.Magnesium />). 



No. I. Bluish green. 



No. 2. Lighter bluish-green. 



No. 3. Same as No. 2. 



No. 4. Ruddy black. 



Blue {near the blue lithium). 



No. I. Griiss green. 

 No. 2. Lighter grass green. 

 No. 3. Same as No. 2. 

 No. 4. Ruddy black. 



Violet {all the violet of the spectrum). 



No. I, 2, and 3. Light violet. 

 No. 4. Darker violet with a suspicion of red. 

 When a red a little below the red lithium line was employed, 

 all the groups appeared dark red, and as in the 3 sensation 

 theory this part and the violet are simple sensations, the results 

 obtained in these ia^t, were to be expected. 



The next two series an- interesting, observations being made 

 in while light compounded by the mixture of two simple 



colours. , , ■., 



Mixture of red and !^fen to make white. 



No. I. Indigo-blue. 

 No. 2. Reddish orange. 

 No. 3. Same as No. 2. 

 No. 4. Dai ker orange. 



.Wi.xlure of yellow and blue to wake white. 



No. I. Sky blue. 



No. 2. Sage green. 



No. 3. Same as No. 2. 



No. 4. Bluish-black (perhaps black). 

 These results were confirmed by an independent observer 

 When the lotalion was reversed the same order of olours was 

 observed, but in the reversed order. These observations seem 

 to confirm the original opinion I had formed regarding these 

 phenomena. , , , , ... 



Hearing in mind that none of the observed colours in the 

 lines are pure colours, but mixed with a cert.iiii quantity ol 

 white and are seen on a more or less dark groun.l, ihen il the 

 order' of per>isiency of the three culour sensations be violei 

 (blue) urcen.red, the results would be as given .ibove. Should 

 Ihis be so the velociiy of rotation must alier the position of the 

 colours seen in white light, the violet being the last to be seen 

 on No. I when rotaied more rapidly, and this is the case. Ihe 

 effect of contrast also has to be taken into .iccount. 



I have made a good many more .xperiments under varying 

 conditions of position and dimensions ol lines and proportions of 

 black to white; and it seems possible that this toy, when 

 modifte.l, may be adapted to give valuable inform.aimn as regards 

 certain problems in colour vision. W. i)K W. .XBNE^. 



We have recently made a few experiments bearing on tlic 

 phcnomcna exhibited by Mr. C. E. Benham's artificial spectrum 

 ion (see NATURE, November 29, 184. P- I I3t. and the ex- 

 plknalion of them sug,;e.sied by Prol. Live.ng (Nature. 

 December 13, 1894, p. 167). and have .duained results which 

 we believe to be novel and of some scentihc inicrcst. 



In ihe first place, if I'rof. I.iveing's expl.inanon he correct, 

 there seems to be no reason why the same ctfects should not be 

 obiained with broad bands ins.e.al of lines, the h"";>; '';";^: 

 diawn in precisely the same manner as the lines upon the white 

 half ol the disc. , ., ,. ■ . 



It appeared to us of some importance to determine if this 

 were re.Mly Ihe case, as wc thought Ihe effects obl.iined with 

 Mr. Benham's top might po-sibly be due to ■'[•^■''">">" ''^^'''^ 

 different in amount for the differeni col.mrs, whih- the c .ange n 

 colour with reversion indirection of rotation w.-,s p.esiimably due 



,0 the black lines succeedinj;, upon a given p.iri of ihe reina, a 

 previously while ground in the one case and a previously bbck 

 one in the other. 



NO 1317. VOL. 51] 



