312 



FRAUNHOFER FRECKLES. 



rately. Fraunhofer invented a machine which obvi- 

 ated this difficulty, and rendered the surface more 

 accurate tlian it was left by the grinding. He in- 

 vented, also, other grinding and polishing machines, 

 and introduced many improvements into the manu- 

 facture of the different kinds of glass used for optical 

 instruments, and which he found to be always injured 

 by flaws and irregularities of various sorts. 



In 1811, he constructed a new kind of furnace, 

 and, on the second occasion when he melted a large 

 quantity, found that he could produce flint-glass, 

 which, taken from the bottom of a vessel containing 

 two cwt. of glass, had the same refractive power as 

 glass taken from the surface. He did not again 

 succeed so well for some time ; yet he continued to 

 study the causes of his failure, always melting at once 

 four cwt. He found that the English crown-glass 

 and the German table-glass both contained defects, 

 which occasion irregular refraction. In the thicker 

 and larger glasses, there would be more of such de- 

 fects, so that, in larger telescopes, this kind of glass 

 would not be fit for object-glasses. Fraunhofer 

 therefore made his own crown-glass. The cause 

 which had hitherto prevented the accurate deter- 

 mination of the power of a given medium to refract 

 the.rays of light and separate the different colours 

 which they contain, was chiefly the circumstance that 

 the colours of the spectrum have no precise limits, 

 and that the transition from one colour into another 

 is gradual, and not immediate ; hence the angle of 

 refraction cannot, in the case of large spectra, be 

 measured within 10' or 15'. To obviate this diffi- 

 culty, Fraunhofer made a series of experiments, for 

 the purpose of producing homogeneous light artifi- 

 cially ; and, as he was unable to effect his object in 

 a direct way, he invented an apparatus, which ena- 

 bled him to attain it by means of lamps and prisms. 

 In the course of these experiments, he discovered 

 that bright fixed line, which appears in the orange 

 colour of the spectrum, when it is produced by the 

 light of fire. This line enabled him afterwards to 

 determine the absolute power of refraction hi different 

 substances. The experiments to ascertain whether 

 the solar spectrum contains the same bright line in the 

 orange as that produced by the light or fire, led him 

 to the discovery of the innumerable dark fixed lines in 

 the solar spectrum, consisting of perfectly homogen- 

 eous colours. This was an important discovery. 

 Fraunhofer has described his experiments relating to 

 these discoveries in vol. v. of the Memoirs of the Royal 

 Bavarian Academy, and in vol. Iv. of Gilbert's An- 

 nalen der Physik. The accounts have been translat- 

 ed into several languages. In 1817, he was chosen 

 a member of the academy of sciences at Munich. 



Fraunhofer made other experiments besides those 

 on the reflexion and refraction of the light, particu- 

 larly on the inflection of light, the happy success of 

 which led him to the discovery of the very different 

 phenomena which are produced by the mutual influ- 

 ence of inflected rays : for instance, he was enabled 

 to produce perfectly homogeneous spectra of colours 

 entirely without prisms. As these spectra, which are 

 produced simply by fine threads, perfectly equal and 

 parallel, placed close to each other, contain those 

 dark fixed lines, which he had formerly discovered in 

 the spectrum produced by a prism ; and as, therefore, 

 following the course of the light, the angles could be 

 ascertained with an extraordinary precision, the 

 curious laws of this modification of light could be de- 

 duced with unusual accuracy. (See vol. viii. of the 

 Memoirs of the Bavarian Academy, and Part II. of 

 Schumacher's Astronomical Treatises, in French.) 

 The laws of light, as then known, were such that 

 several hypotheses could be adapted to them. Fraun- 

 hofer, in endeavouring to find a theory which should 



embrace his discoveries, saw that they could be satis- 

 factorily explained on the principles of interference, 

 that is, according to doctor Young's hypothesis of 

 undulation, with certain modifications. Proceeding 

 on these principles, he established a general analyti- 

 cal expression for the new laws of light, from which 

 it appeared that if he were capable of making an in- 

 strument consisting of perfectly parallel threads, so 

 fine that about 8000 would make only one Parisian 

 inch, the phenomena produced by them would be. 

 modified in a way apparently very complicated. He 

 therefore made a new course of experiments, and in- 

 vented a machine for division, which enabled him to 

 produce such instruments with the necessary accur- 

 acy. The results of these experiments, which per- 

 fectly justiiy the theory, were published by Fraun- 

 hofer, in vol. Ixxiv. of Gilbert's Annals of Physics. 

 Until his death, he was occupied with the further in- 

 vestigation of this interesting subject. Several at- 

 mospheric phenomena, which formerly could not be 

 explained according to the laws of light then known, 

 (for instance, halos, parhelia, &c.), were explained 

 on optical principles, by Fraunhofer. A treatise on 

 this subject is contained in Schumacher's Astronomi- 

 cal Treatises. 



We must remark, further, that he made, with his 

 own hands, the instruments which he invented for 

 his experiments, and, at the same time, executed the 

 engravings for his treatises. Some of the most im- 

 portant instruments, either invented or much im- 

 proved by him, and now generally known, are the 

 following : the heliometer ; the ring-micrometer ; the 

 lamp-circular and net- micrometer (described by Fraun- 

 hofer, in No. 43 of the Astronomischen Nachrichtcn, 

 transl. in Philosophical Magazine, March, 1824) ; the 

 grand parallactic refractor, for the university of Dor- 

 pat (see Struve's Description of the great Refractor 

 of Fraunhofer, in the Observatory at Dorpat ; Dor- 

 pat, 1825, folio, with engravings), &c. At a later 

 period, by order of the king of Bavaria, Fraunhofer 

 made a still larger parallactic refractor, the object- 

 glass of which is of twelve Parisian inches diameter, 

 and of eighteen feet focus, which he carried to greater 

 perfection. In 1819, the optic institution, which had 

 become so famous under his direction, was transferred 

 from Benedictbeurn to Munich, where it occupies, at 

 present, about fifty persons. The firm, until 1814, 

 waslltzschneider, Reichenbach and Fraunhofer ; since 

 that year, Utzschneiderand Fraunhofer. Fraunhofer 

 was member of many foreign academies. This dis- 

 tinguished man died June 7, 1826, probably in con- 

 sequence of his unremitted labours and the neglect to 

 take proper care of his physical wants. His grave 

 is near that of Reichenbach, who died a few days be- 

 fore him. The appropriate epitaph Approximavii 

 sidera is inscribed on his tomb. See sketch of his 

 life, by Jos. von Utzschneider ; also the articles Re- 

 fractor, and Utzschneider. 



FRECKLES ; small spots of a yellowish colour, 

 scattered over the face, neck and hands. Freckles 

 are either natural, or proceed accidently from the 

 jaundice, or the action of the sun upon the part. 



Heat, or a sudden change of the weather, will often 

 cause the skin to appear of a darker colour than 

 natural, and thereby produce what is called tan, sun- 

 burn, &c., which seem to differ only in degree, and 

 usually disappear in winter. Persons of a fine com- 

 plexion, and those whose hair is red. are the most 

 subject to freckles, especially in those parts which 

 they expose to the air. The origin of freckles is ex- 

 plained in this way : In the spring, the skin, from 

 the warm covering which the body has had in winter, 



and from various other causes, is peculiarly sensitive. 



The heat of the sunbeams now draws out drops of 



moisture, which do not dry as rapidly as in summer 



