May 26, 1892] 



NATURE 



81 



apparently coincides with Ai— the reflection of A in the 

 upper surface of the upper plate ; and thus, neglecting 

 for the time light which has undergone more than one 

 reflection, we see this AjB^ combination of reflections 

 illuminated by light which has undergone reflection at 

 the two inner surfaces only. 



It is clear that if we substitute for the two glass plates 

 the apparatus generally sold for exhibiting Newton's 

 rings, we can by this simple method view the rings by 

 the light proceeding from the two inner surfaces only. 

 Thus viewed, the central dark spot appears of a rich | 

 velvety black, and the coloured rings very brilliant. The 

 experiment can easily be projected, and the difference in 

 the appearance of the rings on the screen, with and with- 

 out the opaque screens, is very striking. 



The effect of the two screens can be still more simply 

 given by cutting a slit in a piece of blackened cardboard of 

 about the same width as the thickness of one of the glass 

 plates in the rings apparatus ; it is almost needless to 

 state that the cardboard in the region of the upper and 

 lower edges of the slit performs the functions of the 

 screens B and A respectively. In tbis way the backing of 

 the lower glass plaie (to get rid of the reflection from its 

 lower surface) may be avoided ; an obvious advantage 

 when it is desirable to show the interference in the 

 transmitted as well as in the reflected light. 



But the interest of the method does not only lie in its 

 simplicity. Besides affording an easy proof that the 

 rings are caused by light reflected at the inner surfaces of 

 the plates, it also gives a method of seeing and possibly 

 differentiating the interference curves produced by light 

 which has undergone only one reflection, i.e. the rings 

 commonly known as Newton's, from the curves produced 

 by the interferences of waves which have undergone two 

 reflections or more (and these last, so far as I know, can 

 only be shown by this method) ; for if, using the ring 

 apparatus and a single opaque screen, say 3 inches X 

 ;'; inch, we look into the central reflection (A.,) carefully, 

 two sets of rings, intersecting, can be seen. These cannot 

 be due to light reflected at the points whence the rays 

 which form the primary rings are reflected — by what has 

 gone before. 



To indicate, without attempting for the present any 

 further analysis, how some of the other interference 

 systems may be rendered visible : — Tate a strip of 

 blackened cardboard, say 8 inches X 2| inches, and view 

 its reflections in the Newton's rings apparatus. C (see 

 Fig. 4) being the lower portion of this new screen, its 



C; 



Cs 



c% 



C4 



Cs 



reflection will be seen to consist of a number of shaded 

 strips, Ci, C.^, Cj, &c. ; and in each of these will be 

 evident different mterference curves (plainer, of course, 



NO. 1178, VOL. 46] 



when monochromatic light is used) ; in Ci the primary 

 rings ; in C, two series of rings crossed ; m C3 still 

 more complicated forms, and so on ; each set fainter 

 than the last, the light to which it is due having under- 

 gone more reflections than its predecessor. The method 

 suggested for the experimental analysis of these inter- 

 ference systems can only be sketched roughly here. It 

 is, by the use of a second screen, possibly a third, so to 

 combine the reflections of the screens with observations 

 of the consequent alteration in the interference curves, as 

 to completely verify the results a mathematical analysis 

 of the problem would predict. T. C. Porter. 



/EAN SERVAIS STAS. 



FEW, if any, among the men of science of the present 

 day have at once done such important work and 

 earned so little popular recognition as Jean Servais Stas. 

 The names of Faraday, Liebig, Dumas, Darwin, have 

 become household words beyond the laboratory and the 

 lecture theatre, and are frequently taken in vain by the 

 purveyors of "science for the million." But, whether 

 among the "classes" or the "masses," if we mention 

 Stas we are apt to be asked, Who was he ? What has he 

 done? If we mention his determination of the atomic 

 weights, we have to follow this statement up with a popu- 

 lar lecture on stochiometry, and are then told that there 

 is not much in it. 



Stas was born at Louvain, on August 21, 1813. Like 

 many young men of scientific tastes in the earlier part of 

 the century, he entered upon the study of medicine, and 

 graduated as M.D. But, feeling himself strongly drawn to 

 chemical research, he came to the conclusion that the life 

 of a practising physician was not his true sphere. So 

 early as 1835 he undertook, in conjunction with his friend 

 De Koninck, an investigation of the root-bark of the 

 apple-tree, and discovered phloridzine, an interesting 

 crystalline body. However, at the outset he merely 

 succeeded in obtaining this body in its pure state and in 

 ascertaining its behaviour with reagents. He decided 

 to go further, and to study the constitution and trans- 

 formations of phloridzine. To this end he stood in need 

 of further instruction. But the methods of organic in- 

 vestigation were at that time little advanced. The art 

 of research was taught only by Liebig at Giessen, and 

 by Dumas at Pans. Stas made choice of Dumas, and 

 after overcoming endless difficulties, was admitted as a 

 pupil in the laboratory of that distinguished Academician. 



Here, he resumed the study of phloridzine, and soon 

 succeeded in determining its formula, and those of its 

 principal derivatives. He ascertained that in contact with 

 acids, phloridzine was split into glucose and phloretin, 

 thus belonging to the class of glucosides, bodies the 

 prototype of which had been discovered by Liebig and 

 Wohler in amygdaline. Berzelius, a man by no means 

 lavish of praise, declared that " from an investigator who 

 has carried out such a research much may be expected." 



Impressed with the ability of his pupil, Durinas re- 

 quested him to undertake a series of investigations in 

 concert with himself. The first of these researches was 

 the examination of the action of potassa-lime on alcohols. 

 They determined that, without exception, alcohols were 

 transformed into corresponding acids. By their powers 

 methylic alcohol yielded formic acid, and ethylic alcohol 

 yielded acetic acid. Fusel-oil gave a valerianic acid 

 exactly agreeing in its properties with the natural valeri- 

 anic acid — a discovery of great importance consider- 

 ing the paucity of synthetic organic compounds then 

 known. In conjunction with his master, he ascertained 

 the molecular weight of valerianic acid by a determination 

 of its vapour density and by its conversion into tri- and 

 tetra-chlorvalerianic acid, thus justifying their joint behef 



