92 



NATURE 



[May 30, 1872 



will be contracted and extended In opposite quadrants until at 

 45° they are divided liy two diagonals, on each side of which the 

 colours are complementary. Beyond 45" the rings begin to 

 coalesce, until at 90° the four quadrants coincide a^ain. During 

 this movement the centre has changed from bright to dark. If 

 the motion of the analyser be reversed, tlie quadrants which 

 before contracted noiv expand, and tviv -■(/•jvi. Again, if the 

 crvstal {say positive) be replaced liy another (say negative), the 

 effect on the quadrants of the ring; will be reversed. This 

 method of examination therefore affords a test of the character, 

 positive or negative, of a crystal. 



A similar ])rocess applies to biaxal crystals ; but in this case 

 the diagonals interrupting the rings are replace! by a pair of 

 rectangular hyperbolas, on either side of which the rings expand 

 or contract, and the effect is reversed either by reversing the 

 motion of the analyser, or by replacing a positive by a negative 

 crystal, or r'Av 7vrsS. The experiment may then be made in 

 biaxal crystals, by turning the analyser slightly to the right or to 

 the left, and observing whether the lings advance towards or 

 recede from one another in the centre of the field. In particular, 

 if, polariser and analyser being par.allel. the plate A have its axis 

 inaN.E. direction to a person lojking through the analyser, 

 the plate B its axis in a N.W. direction, and the crystal be .so 

 placed that the line joining the optic axes be N.S., then on turn- 

 ing the analyser to the right the rings will advance to one another 

 if the crystal be negative, and recede if it be positive. The 

 mathematical expression for the intensity of the light at any point 

 P is in this case 



,5(1 + sin. 2j cos. fl + sin. 2 /' cos. 27' sin. 6), 

 where /' is the angle between the principal section of C through 

 r and the principal section of 13, and j the angle between the 

 ]irinc!pal sections of B and the analyser. This shows that when 

 the polariser and analyser are parallel or crossed at 0° or 90', 

 and consequently/ = 45° or 135", the expression is independent 

 of/', /.(■., the intensity is the same throughout circles about the 

 centre, but that when the polariser and analyser are crossed we 

 have an expression of the form 



1 (i ± sin. 2 /' sin. B), 

 the sign of the second term d^'pending upoa the direction in 

 vvtiich the analyser has b^en turned, and aho upoa the sign of 

 6, that is, upon the character (positive or negative) of the crystal. 



The dispersion of the planes of polarisation effected by the 

 pas>age of plane polarised light through a pUte of quartz cut 

 perpendicular to the axis may be rendered visible by interposing 

 such a plate of quartz between the polariser and a uniaxil or 

 biaxal crystal, when the analyser is at 90°, i.i:, when dark 

 b.ushes are formed. In this case the brushes ceise to be black, 

 and are tinged throughout with colour. The analyser mu>t, 

 however, be turned back or forward, according as the quartz be 

 right-handed or left-handed, in order that it may cross in succes- 

 sion the planes of polarisation of the flifferent coloured rays, and 

 so produce the most vivid effects. The dispersion of the brushes 

 by a plate of quartz may, howtvsr, be studied by employing an 

 additional polariser and quartz plate between the source of hght 

 and the whole system previously used. By turning this polariser 

 round we extinguish each ray of the speCLrum in turn, and tint 

 the whole field with the complementary colour. Tne brushes 

 will then appear to revolve aboat their centre i as the tints vary 

 continuously from one end of the spectrum to the other- If the 

 polariser be turned still farther round, the tints which had 

 changed continuously from red to violet, or -I'ice vcrsd, change 

 suddenly from violet to red, or ''iiCTcrsa, and the brushes jump 

 suddenly back to their original position. 



This last optical arrangement may he employed to examine 

 the more important phenomena of the dispersion of the optic 

 axes produced, not by a quartz plate between the u«ual polariser 

 and crystal, but by certain biaxal crystals themselves. 



BOTANY 

 The Leaves of Drosera 

 In a recent note to the Paris Academy of Sciences, M. 

 Zlegler writes as follows : — 



The hairs on the leaves of Droseras exude at their extremity a 

 small drop of glue, by which insects are caught. Whenever an 

 insect becomes attached, the exterior threads bend over it, 

 covering it like the fingers of a hand, and do not straighten again 

 till some days after, when a fresh drop exudes for a fresh prey. 



In studying these remarkable plants, I noticed that all the 

 albuminoid animal substances, if held for a moment between the 

 fingers, acquired the property of making the hairs of the Drosera 

 contract. I also observed that such subst,ince5, when they had 

 not been in contact with a living animal, had no visible action 

 on the hairs. This shows that the simple contact of the fingers 

 communicates to inert animal substances a property which they 

 did not jiossess before. 



These same animal substances thus prepared lost this singular 

 property when they were moistened several times with distilled 

 water, and dried each time in a water-bath. This is a conve- 

 nient mode of preparing the substances for experiment. 



The contraction of the hairs is not caused by animal heat, 

 which the fingers may have communicated to the animal sub- 

 stances, for the hairs contracted equally when the substance 

 hal been cooled before placing it on the leaf. 



The perspiration of the fingers cannot affect the phenomenon, 

 for the property can be communicated to animal substances 

 across fine waxed paper. And the residt is not affected if the 

 substances are first covered with a coating of wax, thus pre- 

 venting any chemical action of soluble matters which the 

 animal substances may contain. 



A living animal thus communicating by simple contact new 

 physical properties to an inert body, it was important to know 

 whether, by increasing the amount of transmission, we should 

 observe any change in the vital state of the animal. Some rab- 

 bits were enclosed in light wooden cages. These were of such a 

 size that their sides were always in contact with the hairs of tie 

 animal at one part or other. To the outside of the cage were 

 attached bigs of cloth or paper, containing (for each cage) two 

 kilogrammes oi dried serum (albumen from blood). Other rab- 

 bits were placed in exactly similar cages, but without the albu- 

 men. Their food consisted of 25 grammes of hulled oats every 

 twenty four hours, with cabbage leaves at discretion. 



At the end of some days, the rabbits that had been in contact 

 with the albumen became diabetic in a high degree (though 

 without saccharine matter) ; the urine was given in normal 

 quantity, but the loss in ammoniaco-magnesian phosphate was 

 very great, and these rabbits deteriorated and lost weight. The 

 other rabbits, which had not been in contact with albumen, re- 

 mained in their normal state, and even gained weight. 



It was interes'ing to ascertain if the avidity of the Drosera for 

 insects was insatiable, and to find what would be the effect on it 

 of increasing the contact of a living animal. Some dozens were 

 accordingly placed, with a small clod of earth and sufficient water, 

 in light platinum capsules. The?e capsu'es were each placed in 

 a sheath containing blood albumen, which h.ad previously been 

 held for half an hour in the hand. At the end of twenty-four 

 hours all these Droseras had become quite insensible to insects 

 and to organic animal bodies mxlified by living contact. The 

 properties of these plants were reversed, and strange to say, their 

 liairs were found to contract under the influence of organic mat- 

 ters which had previously been in contact with paper packets (of 

 double or triple envelope) containing sulphate of quinine. Organic 

 matters influenced in this purely physical manner hy sulphate of 

 quinine have no contractile action on the hairs of the Drosera in 

 their nor.-nal state. The plants whose physical properties have 

 been reversed by the influence of albumen in the above way, 

 could be restored to their normal state by placing them for 

 twenty-four hours with the platinum capsule on a packet of sul- 

 phate of quinine. This method may be adopted whenever, from 

 any cause, the leaves have become insensible to insects. In 

 every case the contraction of the hairs is always slow ; it com- 

 mences visibly in about a quarter of an hour, and is often not 

 complete till after several hours. 



Among vegetable matters seeds only are impressible in the 

 way referred to, and the experiments mentioned (which weie 

 made with albuminoid anim.al substances) may be repeated 

 with these. 



Nature of Diatoms 



I.\ a recent essay by Prof. Adolf Weiss, of Lemburg ("Zum 

 Baue und der Natur der Diatomaceen"), it appears to be de- 

 monstrated that the siliceous investment of these little plants has 

 cellulose f)r its base. The silex is infiltrated to a variable 

 extent in the different families, and the mode of its deposition 

 can to a certain extent be ascertained by examination with 

 polarised light. In opposition to the opinion hitherto generally 

 admitted, I'rof. Weiss shows that the siliceous coal is capable of 



