276 



NATURE 



\Jaiu 25, 1877 



part of the field in the two cases. The expressions will 

 be found in Verdet (Oeuvres, tome vi. p. 1 10), who further 

 remarks that, if the two sections are parallel, the addition 

 of the second is equivalent to an augmentation, or a dimi- 

 nution of the thickness, according as the two plates are 

 both positive or both negative, or are one attractive and 

 one repulsive. If the principal axes of the plates are 

 perpendicular, the addition of the second plate is equi- 

 valent to a diminution of the thickness when th« plates 

 are of the same sign, and to an augmentation of thickness 

 when they are of opposite signs. 



When, as in the case proposed, the second plate is a 

 wedge, the effect of the combination will be the same as 

 if the flat side of the concave plate were cut away wedge- 

 wise, but in direction opposite to that of the actual wedge. 

 Optically, then, the bottom of the concavity will be thrown 

 towards the side on which the combination is optically 

 thinnest ; i.e., on which the actual wedge is thinnest. 



The sliding of the wedge will not alter the displacement 

 of the centre, which is dependent on the angle, and not 

 on the thickness of the wedge, but it will alter the total 

 thickness of the compound plate, and consequently the 

 diameter of the circles. 



In addition to the above, I may mention another piece 

 devised and constructed for me by Mr, C, D. Ahrens. 

 This consists of two quartz cones, one hollow, the other 

 solid, fitting into one another ; one cone is of right- 

 handed, the other of left-handed, quartz, and the axis of 

 each is parallel to that of the crystal. The polarisation 

 figure due to this combination is of course a series of 

 concentric circles, which expand or contract when the 

 analyser is turned in one direction or in the other. 



If the field of view be examined at various distances 

 from the centre, it will be found that there is a distance, 

 viz., where the right and left-handed canes compensate 

 one another, at which there is no colour, but only an 

 alternation of light and darkness. In the immediate 

 neighbourhood of this the red and orange assume the 

 brown and drab hues due to low illumination, in accord- 

 ance with Helmholtz's experiments ; beyond this the 

 colours are more brilliant ; while at still greater distances, 

 where the thickness of one cone much exceeds that of 

 the other, the colours become more pale. 



Combe Bank j W. Spottiswoodk 



MUSEUMS 



THE subject of Museum management and arrange- 

 ment having lately been commented upon by Prof. 

 Flower in a lecture delivered at South Kensington 

 Museum, and printed in Nature of December 14, 28, 

 et seq., and also by Prof. Boyd Dawkins in an address to 

 the Manchester Literary and Philosophical Society, 

 noticed in Nature of December 7, it may not be an 

 inopportune time to suggest to those who have the 

 management of these institutions the desirability of their 

 mutual co-operation in order to develop them to their 

 fullest extent. The great progress which has been made 

 during the past few years in the establishment of mu- 

 seums in the various provincial towns of this country is 

 highly creditable to those who have assisted in the move- 

 ment, and the influence which they might have, if pro- 

 perly utilised and developed, on the education and intel- 

 lectual progression of the people, gives them a forcible 

 claim to national and individual support. 



There is no doubt that the present financial support 

 given to museums is totally inadequate to maintain them 

 in an efficient state, and we hope to see this remedied to 

 some extent in the next session of Parliament, by the 

 adoption of Mr. A. J. Mundella's Bill for Increasing the 

 Library and Museum Rate, the penny rate not realising 

 sufficient money, except in large and wealthy towns, to 

 serve the purpose for which it is intended. Mr. Mun- 



della's Bill gives the power to levy a rate, not exceeding 

 2d. in the pound, for the purpose of establishing and 

 maintaining free libraries and museums, and in those 

 towns where there is no museum, but only a library or 

 libraries, the rate not to exceed i\d. in the pound. This 

 slight increase would not press heavily on any section of 

 the ratepayers, while it would enable many of our libra- 

 ries and museums which are now languishing for want of 

 funds to go on in their wide sphere of usefulness with 

 increased vigour and zeal. 



The important conference of the mayors of towns and 

 chairmen of museum committees, which was held in Bir- 

 mingham on the 5th instant, to discuss and urge the 

 claims of museums and galleries of art to some of the 

 surplus funds remaining from the International Exhibi- 

 tion of 185 1, and also to the duplicates which are stored 

 away in Government collections, is a step in the right 

 direction, and we hope that it will be productive of good 

 results. 



With the mete general diffusion of education among 

 the great masses of the people which it is hoped will 

 result from the passing of the recent elementary educa- 

 tion acts, the class of readers and students will be 

 greatly enlarged, and we might naturally look for some- 

 thing being done by the Government to meet the increased 

 demand for books and objects of study which is likely to 

 follow ; and we see no reason why museums themselves 

 should not be occasionally converted into schoolrooms 

 where teachers could bring their zoological, geological, 

 and other natural science classes, and find well-arranged 

 material for illustrating their lessons. Of course we take 

 it for granted that these sciences will eventually be taught 

 in all elementary schools under Government control. 



With regard to the preservation and arrangement of 

 specimens in museums, those who had the pleasure of 

 listening to Prof. Flower's lecture cannot have failed to carry 

 away some useful suggestions, and his remarks, together 

 with those of Prof. Boyd Dawkins on the neglected and 

 unsatisfactory state of many of our museum collections, 

 are well worth the consideration of those in charge of 

 them. What ought fairly to be the aim and scope of a 

 provmcial museum is a question which each town must 

 to a great extent answer tor itself, as it must depend on 

 the resources of the neighbourhood and on the facilities 

 possessed for obtaining certain classes of objects ; and 

 curators would do wisely to content themselves with doing 

 only what can be done thoroughly and well, be it ever so 

 little, and not to accept objects simply with the vie w of filling 

 empty cases. W^e are well aware that curators are not 

 always responsible for the incongruities which get into a 

 museum, and if the refusal of objects were always left 

 with them, we should not have museums sinking into 

 advertising establishments or mere curiosity shops. We 

 do not, however, intend now to go into this subject, but 

 in order that all the important matters connected with the 

 work of museums may receive full and careful considera- 

 tion, we would suggest that an association be forrred, to 

 consist of curators and others engaged in the arrangement 

 of museums. Such an association need not in any way 

 interfere with those now existing, as there is a sufficiently 

 wide field for discussion and action included in the work 

 and development of museums without treadmg on the 

 ground occupied by other associations. By holding pe- 

 riodical meetings and constantly changing the place of 

 meeting from town to town, the various museums of the 

 kingdom could be inspected, and their contents and plan 

 of arrangement discussed and criticised. Friendly com- 

 munications would thus be opened among all museums, 

 and exchanges could be arranged to their general ad- 

 vantage. Much might be said as to the necessity and 

 work for such an association, but we content ourselves, 

 for the present, with suggesting it, and now leave the 

 matter to be taken up by those most intimately concerned. 



E. H. 



