Feb. 26, 1874J 



NATURE 



323 



Were there any such laws of elevation and subsidence as Mr. 

 Howortli maintains, the respective regions of elevation and of 

 subsidence would have continued the same since the consolida- 

 tion of the earth : but this is contradicted by the commonest 

 facts of stratification, which show that elevation and subsidence 

 have everywhere alternated with each other. 



Joseph John Murphy 



Old Forge, Dunmurry, co. Antrim 



The Use of Terms in Cryptogamic Botany 

 As no specialist in Algology has replied to the inquiry of your 

 correspondent "D. R.," in Nature for January 15, I may be 

 permitted to quote for his information the following from thearticle 

 "Nucleus, "in the "Treasuryof Botany "from the pen of the author 

 of the "Introduction to Cryptogamic Botany": — "In Alg.x 

 the term is applied to the fructifying mass of the Rhodospcrms, 

 whether contained in a single cell or in a compound cyst or con- 

 ceptacle, the word nucleoli being used when there is a group of 

 nuclei." The instance alluded to by your correspondent is, un- 

 fortunately, not the only one in which the terminology of cryp- 

 togams is in a state of most perplexing confusion. 



Alfred W. Bennsitt 



A Lecture Experiment 



The condensation of liquid in the form of vapour into minute 

 globules, and the production of a shower of rain, may be very 

 well illustrated for class purposes in the following manner : — 



Place about an ounce of Canada balsam in a Florence flask, 

 and let it boil. At the top of the llasli clouds of globules of 

 turpentine will be seen hovering about, altering in shape very 

 much like sky-clouds, and the globules are large enough to be 

 visible by the naked eye. If a cold glass rod be gradually intro- 

 duced into the flask, these clouds may be made to descend in 

 showers. By the adaptation of a lime-light the whole process 

 could be shown on a screen. Lawson Tait 



TODHUNTER ON EXPERIMENTAL 

 ILL USTRA TIONS 



I fidelibus, et qua: 



Segntus i 



quam quse suntoculis subje 



ipse sibi tradit spectator. 



THE following is, as nearly as I can recollect, the 

 substance of a few remarks which I felt myself 

 compelled to make to my class in a recent lecture. 1 

 had exhibited and described Hope's apparatus for 

 showing the maximum density point of water, and pro- 

 ceeded to say : — 



Now that the freezing mixture has been applied, 

 my assistant will from time to time record on the 

 black-board the simultaneous indications of the two 

 thermometers, and will recall our attention to the experi- 

 ment as the critical period approaches. You must, how- 

 ever, in this form of experiment take for granted his 

 fidelity and accuracy in reading and recording. 15y means 

 of a somewhat cumbrous application of optical processes, 

 it would be easy to project upon a screen images of the 

 thermometers, in such a way that each of you might see 

 for himself the course of the phenomenon. But the ther- 

 mo-electric method, whose principle I have already 

 explained to you, is at once far easier of application, and 

 in its indications more directly expressive. This I will 

 show on another occasion. For the present you must 

 rely on the observations to be made for you by my assis- 

 tant. Yet I have no doubt that all of you will allow that 

 the exhibition of the experiment, even in this imperfect 

 manner, wonderfully assists you in understanding its 

 nature. 



This leads me to mention that a very decided opinion 

 against the use of experimental illustration has been 

 recently pronounced by one of the most erudite and 

 voluminous of British mathematicians ; my own former 

 tutor, Mr. Todhunter, whose name and many of whose 



works must be familiar to most of you. Such a man 

 speaks, deservedly with authority, on many points ; and 

 therefore his dicta upon a point with which he shows 

 himself to be totally unacquainted are especially dange- 

 rous. And I feel that it is my duty to paint out to you, 

 aiid warn you against, errors or absurdities connected 

 with physics, whenever they come from one whose state- 

 ments are, on other grounds, worthy of attention. I shall 

 not trouble you with the whole passage I refer to in Mr. 

 Todhunter's "Conflict of Studies," but merely read to you 

 a sentence or two of the most astounding part of it. I 

 premise that though he is speaking of the teaching of 

 physical science m schools, his observations apply (if they 

 have any basis whatever) to science-teaching in general. 



" It may be said that the fact makes a stronger impres- 

 sion on the boy through the medium of his sight, that he 

 believes it the more confidently. I say that this ought 

 not to be the case. If he does not believe the statements 

 of his tutor— probably a clergyman of mature knowledge, 

 recognised ability, and blameless character — his suspicion 

 is irrational, and manifests a want of the power of appre- 

 ciating evidence, a want fatal to his success in that 

 branch of science which he is supposed to be cultivating." 



Verbal comment on this would be altogether super- 

 fluous, and the only ptactical comment I am disposed 

 now to make is to proceed at once to farther experi- 

 f«(r«/,z/ illustrations of the subject before us. 



P. G. TAIT 



POLARISATION OF LIGHT* 



V. 



'T'HE conversion of plane into circularly polarised light 

 -•- may also be effected by total reflexion. If plane- 

 polarised light traversing glass be incident upon the inner 

 side of the limiting surface at any angle at which total re- 

 flexion takes place, it may be considered as resolved into 

 two plane-polarised rays, the vibrations of one being parallel 

 and those of the other perpendicular to the plane of 

 reflexion ; and there is reason to believe that in every 

 such case a difference of phase is brought about which 

 for a particular angle in each substance (in St. Gobain 

 glass it is 54° 30') it has a maximum vakie of one-eighth 

 of a wave-length. And if the original plane of vibration 

 be inclined at an angle of 45° to 

 that of reflexion the amplitudes 

 of the two vibrations, into which 

 the reflected vibrations are sup- 

 posed to be resolved, will be 

 equal. A full discussion of the 

 mechanical causes which may be 

 considered to effect this dift'erence 

 of phase would carry us deeper 

 into the more difficult parts of 

 the Wave Theory than would be 

 suitable in this place. But if 

 we accept the fact that the 

 above-mentioned effects result, 

 when polarised light (whose plane of vibration is in- 

 clined at 45" to that of reflexion) is reflected at a 

 proper angle ; then the following construction will be 

 readily understood. Take a rhomb of glass, a, l>, c, d, 

 Fig. 14, whose acute angles are 54" 30' ; a ray incident 

 perpendicularly to either end will undergo two total 

 internal reflexions at the sides, say at / and s, and will 

 emerge perpendicularly to the other end. These two 

 reflexions will together produce a retardation, as described 

 above, of one-fourth of a wave-length. And if the ray be 

 originally polarised and its plane of vibration be inchned 



* Continued from p. 285. 



Fig, T4. 



