SCIENCE. 



IMPROVED THERMO-ELECTRIC APPARATUS. 



At a meeting of the Physical Society on April 24, several 

 papers were read by Mr. R. H. Ridout, F.C.S., including 

 one on an Improved Thermo-electric Apparatus, of which 

 the following is an abstract : — 



Whilst most instruments of research have undergone a 

 process of developments the beautiful instrument of Melloni 

 does not appear to have, progressed since the day of its in- 

 ception. Much annoyance arises from the pile and galvan- 

 ometer being separate, and it is a very common occurrence 

 for a pile to be used with any galvanometer that comes first. 

 In reality they are parts of one instrument, and should there- 

 fore be fixed to the same base-board. Treating them as 

 parts of one whole, many defects are to be found in theory 

 p.nd construction, and also in the mode of using. I have 

 made a critical examination of each part, and embodied the 

 improvements in an instrument which, with the assistance 

 of Mr. Browning, combines great delicacy and simplicity. 



The defects in the theory of the pile are, that the essential 

 or internal resistance must always be much less than the 

 external resistance, and from the low tension of the current, 

 t'.\e disparity cannot be wiped out by using a great length in 

 the galvanometer. In practice the faults are , — (a), the 

 junctions are too deep, and cause short circuiting ; {/>), the 

 bars are too long and give unnecessary resistance ; (c), they 

 are too numerous ; (</), the junctions are too slender ; (A the 

 mass of matter to be heated is too great. These are rem- 

 edied (e, a, and d) by placing the bars in glass tubes, con- 

 necting with them plates of copper ; (b), bars made half 

 usual length ; (c), a single pair only is used. 



The defects of the galvanometer are: — (a), the space near- 

 est the needles is not utilized by the wire ; (/'), the needles 

 are not of the best shape ; (c), the suspension is trouble- 

 some. The remedies are : — (a), the wire is made into flat 

 ribbon, and wound in one bobbin, and the needles mounted 

 so as to permit this ; (b), the needles are flat oblong plates, 

 taken from the same piece of steel, and magnatized in one 

 piece ; an agate cup and pivot remedy (c). 



In manipulation the faults are : (a), the several parts are 

 not mutually adapted ; (b), junctions by different metals 

 are exposed ; (c), the pile and galvanometer are connected 

 first, when, in reality, they should not be connected till the 

 pile has been exposed, or else the current generated ab- 

 stracts the heat from the hot side, and lowers its temper- 

 ature. 



In the complete instrument, as made by Mr. Browning, 

 the pile consists of a pair of elements >£in. long, the copper 

 connections being circular plates i-iooin. thick, and %'m. 

 diameter. The pile is supported by its thick copper ter- 

 minals above the galvanometer, which consists of a flat 

 copper ribt)on, making about 20 turns round a pair of 

 astatic needles, iin. long, and J^in. broad, supported on an 

 aluminium frame, and resting on a fine pivot by an agate 

 cube. A contact key is placed at one side, and makes the 

 only connection in the middle of the instrument. The 

 whole is inclosed in a glass shade, having a perforation at 

 the height of the face of the pile. A glass cone protects 

 the front face from the extraneous heat, and a glass cap the 

 back one. A directing magnet placed above the pile er.a- 

 bles the readings to be taken in any position. 



The source of heat being placed in front of the pile, the 

 shade is turned round till the hole is in the axis of the pile, 

 and left exposed for say 30 seconds. Contact is then 

 made, when the deflection of the needle indicates the 

 strength of the current very nearly. A very distinct deflec- 

 tion may be obtained from a person standing 6ft. from pile, 

 and a common candle affects it at 3ft. Further, it shows 

 that the \ r alls of a room are of different temperatures, and 

 in any clear weather radiation into stellar space is very 

 nt. The whole thing can be put ready for experiment 

 in half a minute, while, with other forms, the necessary 

 adjustment usually takes more time than the experiments. 

 The same form ol galvanometer is also supplied separ- 

 ately. 



ERRORS OF REFRACTION IN 

 MICROSCOP1STS. 



THE EYES OF 



BY JOHN C. MORGAN, M. D. 



It will, I think, be at once admitted that the requirements 

 in construction and adjustment of glasses, and the results 

 of work done, must vary greatly with individualities of the 

 workers' eyes. 



One of the most important, but least thought of, is astig- 

 matism* a condition known to oculists as a common 

 cause of occipito-cervical headaches, sometimes so severe 

 as to be considered due to grave hyperaemia of the brain, 

 or to " brain-fag," etc. 



This defect consists in a diversity of curvature; hence, 

 of refraction of one meridian of the cornea, as, for instance, 

 the vertical, with another meridian (horizontal). One of 

 these meridians may be " far-sighted," the other " near- 

 sighted," or the difference may be more moderate. 



Some slight degree of this is quite common, as many of 

 your readers will discover on viewing a black line at a con- 

 venient distance in these and other positions. In one it 

 will look black and sharp ; in another, at a right angle, 

 pale, ill defined, and as if the rays were cut off by a long- 

 itudinal slit in a diaphragm. Such a slit, turned in various 

 positions, has a curious effect, illustrating the influence of 

 loss of the rays. Astigmatism similarly affects vision ; 

 only, in this, dispersion is the immediate cause of loss. 



Another and very simple test of astigmatism is " the 

 point of light" — e. g., a gas flame, reduced to its smallest 

 dimensions (of the yellow), when, to a normal eye, across a 

 large room, it appears as a round point ; but not so to 

 astigmatic or to other abnormal eyes. Dispersion of rays 

 results from imperfect focussing ; and the object seems 

 larger in consequence (but less bright). If this dispersion 

 be only in one meridian of the eye (astigmatism), the ap- 

 parent enlargement will be in exactly the same position, and 

 the image will be long, not round, and thus the individual 

 may note the precise angle in which a cylindroid lens 

 must be worn, for "correction," and the restoration of the 

 round image. If this meridian be short-sighted, the 

 cylindroid must be concave ; if far-sighted, convex. 



The experiment may be varied by using a dark card, 

 with a }i inch round hole in it ; when placed before a win- 

 dow, strongly illuminated, the point of light appears, of 

 course, and it is more accurate in shape than the flame. 



One point more. Spasm of the focussing apparatus 

 (called " spasm of accomodation ") may derange the spher- 

 icity of the eye, and so affect vision. Strained vision is 

 liable to this. On the other hand, the same apparatus may 

 be paralyzed, and ordinary vision deficient, whilst the fo- 

 cussing of the microscope entirely corrects it. 



A linear marking, long or short, on a diatom, or a scale, 

 or a cell, must suffer the same variation in divers positions 

 after the passage of the rays through the best glasses. 

 Some of the disputes as to these may be traced, doubtless, 

 to this cause ; and probably may lie set at rest by the use 

 ol astigmatic spectacles with the microscope. 



These are merely lenses of prescribed cylindrical curva- 

 ture, whose axis is placed in the position of the abnormal 

 corneal meridian, whereby its curvature is corrected. The 

 general effect is to render the whole cornea practically 

 spherical in form. 



Astigmatism has been an injury to painters, as Turner, 

 whose later pictures (the power of accommodation, or self- 

 correction, being lost with age) are discovered to be dis- 

 torted in consequence ; the tendency being to exaggerate 

 the size of the paler dimension in painting it. 



On the contrary, in microscopic drawing, as with the 

 camera lucida, the improperly pale line will be perpetuated, 

 and the perspective misrepresented ; and distortion of 

 dimensions generally may be perpetrated by the most care- 

 ful observers, and endless disputes may thus arise. — Ameri- 

 can Journal of Microscopy. 



* From the Greek, a % f>rivativt t and stigma, a jioint — want of focal 

 point. 



