30 



CAUTERETS 



CAUTION 



bola, any ray falling on the mirror parallel to the 

 axis is reflected so as to pass exactly through the 

 focus. For other mirrors it is approximately true 

 only when the breadth of the mirror is very small 

 in comparison with its radius of curvature. When 

 the breadth of the mirror is large in comparison 

 with its radius of curvature there is no definite 

 image, even of a luminous point. In such cases 

 the image is spread over what is called a Caustic, 

 or sometimes a Catacaustic. 



An example of the caustic is given in the annexed 

 figure for the simplest case namely, that of rays 

 falling directly on a concave spherical mirror, BAB', 

 from a point so distant as to be practically parallel. 



Two veiy near rays, 

 P and Q, will after 

 reflection intersect 

 at C. By finding 

 in this way all the 

 points of intersection 

 of the reflected rays, 

 we get a continuous 

 curve, BCFB', which 

 is the section of the 

 caustic surface by 

 a plane passing 

 through its axis. 

 The curve BCFB' varies of course with the form of 

 the reflecting surface. In the case under considera- 

 tion it is known as an epicycloid. 



The reader may see a catacaustic on the surface 

 of tea in a tea-cup half full by holding the circular 

 rim to the sun's light. The space within the caustic 

 curve is all brighter than that without, as it clearly 

 should be, as all the light reflected affects that 

 space, while no point without the curve is affected 

 by more than the light reflected from half of the 

 surface. The rainbow, it may be mentioned, forms 

 one of the most interesting of the whole family of 

 caustics. 



When a caustic is produced by refraction, it is 

 sometimes called a Diacaustic. No such simple 

 example can be given of the diacaustic curve as 

 that above given of the catacaustic. It is only in 

 the simplest cases that the curve takes a recognis- 

 able form. In the case of refraction at a plane 

 surface, it can be shown that the diacaustic curve 

 is the evolute either of the hyperbola or ellipse, 

 according as the refractive index of the medium is 

 greater or less than unity. 



CauteretS. a fashionable French watering- 

 place in the department of Hautes-Pyrenees, lies 

 3250 feet above sea-level, in the valley of the 

 Laverdan, 5 miles S. of Pierrefitte, the nearest 

 railway station, and 42 SSE. of Pau. The 

 stationary population of the place is only 1260, 

 but it is annually swelled in summer by 15,000 to 

 20,000 visitors, for whose accommodation numer- 

 ous sumptuous hotels and bathing-establishments 

 have been built. It is a good centre and guide- 

 station for ascents among the Pyrenees. The 

 sulphurous springs, twenty-five in number, and 

 varying in temperature from 60 to 131 F., are 

 the most abundant in the Pyrenees ( 330,000 gallons 

 per day), and have been known from Roman 

 times ; though their modern reputation dates from 

 the 16th century, when Margaret, sister of Francis 

 I., held her literary court and wrote much of her 

 Heptameron at Cauterets. 



Cautery ( Gr. kaio, ' I burn ' ), in Medicine, is 

 used of any substance which burns the tissues. 

 (The term 'potential cautery,' as applied to caustic 

 substances, is becoming obsolete.) The actual 

 cautery is an instrument with a head or blade of 

 steel, iron, or platinum, which is heated in a fire or 

 spirit-lamp. In the thermo-cautery (or Paquelin's 

 cautery, from its inventor), the head or blade is 



made of hollow platinum, so arranged that a flame 

 of benzole can be kept burning in its interior. The 

 galvano-cautery consists essentially of a platinum 

 wire which can be heated to any required degree 

 by passing a strong galvanic current through it. 

 The cautery is used for three main purposes in 

 surgery : to produce counter -irritation over an in- 

 flamed part ( see BLISTERS ) ( actual cautery ) ; to 

 check bleeding (actual or thermo-cautery), by 

 slowly destroying the tissues at the bleeding point 

 or surface ; to perform operations, where the tissues 

 to be divided are either very vascular (thermo- 

 cautery), or very difficult of access (galvano- 

 cautery ). See CAUSTIC. 



Caution, in the Law of Scotland, like Guaranty 

 (q.v. ) in England, is an obligation undertaken by a 

 second party, whereby he binds himself, failing the 

 primary obligant, to fulfil his obligation, whether 

 it be of a pecuniary nature or otherwise. Caution- 

 ary obligations are thus essentially of an accessory 

 nature, and cannot subsist apart from the principal 

 obligation. The law of this subject is now largely 

 founded on the Mercantile Law Amendment Acts, 

 1856, which assimilate the laws of England and 

 Scotland, and according to which the creditor 

 may proceed at once against the cautioner, just 

 as if he were a joint obligant, without suing 

 the primary debtor, unless the cautioner has 

 expressly stipulated that this shall be done. 

 The creditor, however, is in every case bound to 

 use proper precaution in retaining and making 

 available securities. He is not, however, bound 

 to make the same full disclosure of material 

 facts as in insurance, and therefore a cautioner 

 should make careful inquiry for himself. Caution- 

 ary obligations are generally gratuitous, being, 

 for the most part, undertaken from motives of 

 friendship ; but it is by no means uncommon for 

 them to be entered into in consideration of a 

 premium paid by the person guaranteed, or by 

 those interested in his fortunes. Where a pre- 

 mium is paid, the transaction becomes a mere 

 insurance of solvency, honesty, or efficiency ; and 

 associations of great public utility (see GUARAN- 

 TEE) have been formed, both in England and 

 Scotland, for the purpose of undertaking to 

 guarantee the fidelity of persons employed either 

 in public or private offices of trust. The tendency 

 of judicial decisions, both in England and Scotland, 

 for many years past, has been to require greater 

 strictness than formerly in the constitution of 

 cautionary obligations ; and under the statutes 

 already mentioned all such engagements must be 

 in writing, subscribed by the person undertaking 

 or making them, or by some person duly authorised 

 by him, otherwise they shall have no effect. If a 

 cautionary obligation is dependent on a condition, 

 it will, of course, be ineffectual unless the condi- 

 tion be complied with. The cautioner may, in 

 general, plead every defence which was competent 

 to the principal debtor, and the extinction of the 

 primary obligation extinguishes the secondary one. 

 The cautioner is discharged by any essential charge 

 being made on the obligation of the debtor, or in 

 respect of the person relied on, without his assent. 

 The statute expressly provides that changes of 

 partnership either of creditor or debtor will ex- 

 tinguish the guarantee. If the creditor gives time 

 e.g. takes bills from the debtor of an unusual 

 currency that will also operate discharge. The 

 discharge of one cautioner, moreover, unless con- 

 sented to by the rest, is a discharge to all. The 

 cautioner is entitled, on full payment, though not 

 on payment by a dividend, to an assignation of the 

 debt and diligence, by which means he comes, in 

 all respects, into the creditor's place ; and more- 

 over, if the solvency or other conditions of the 

 principal debtor should seem precarious, he may 



