Page:Cyclopaedia, Chambers - Volume 2.djvu/600

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REF

Table of Refractions of the Heave?ily Bodies, at the feveral Degrees of Altitude.

Ak.

Refrafi.

Ak. 16

Remit. 3' 26"

Alt. 3°

Refrad. 1' 51"

Alt. + 5

Refrad. l' 9"

Alt. 61

Refrad. 0' 40"

Alt.

77

Refrad. 0' 17''

32' 0"

1

2« 35

17

3 23

3i

1 47

47

' 7

62

39

78

15

2

20 43

18

3 12

32

1 43

48

1 (5


 * 3

37

79

'4

3

'5 44

i?

3 '

33

1 40

49

« 4

<J 4

35

80

12

4

12 2£j

20

2 51

34

1 36

5°

1 2

«|

33

81

11

5

IO 26

21

2 44

35

1 33

5 1

1

66

32

82

10

6

9 8

22

2 38

3«

1 30

52

58

«7

3'

83

8

7

8 8

2 3

2 31

37

1 27

53

5«

68

3°

84

7

8

7 1

a 4

2 24

38

i 24

54

54

69

28

8)

6

9

6 17

25

2 18

39

1 22

55

52

70

26

86

4

10

5 41

26

2 12

40

1 19

5«

50

71

25

87

3

11

5 "

27

2 7

41

1 '7

57

48

72

24

88

2

12

4 4 tf

28

2 3

4 2

1 15

58

4<5

73

23

89

1

«3

4 2 5

29

1 59

43

1 13

59

44

74

21

90

14

4 7

3°

1 55

44

z II

(So

42

75

20

'5 3 5i

1 45

1

7«

18

Tycho Brake will have the RefraBions of die Sun to vanifh at the Altitude of 46 ; thofe of the Moon at 45 s, and thofe of the fix'd Stars at 20 : But Cajjmi has found that they reach even to the Zenith. — Indeed Tycho reprefented all the Refractions lefs than they are ; except the horizontal one, which he made too big : For he makes the horizontal Refraction in the Sun 34' ; in the Moon 33'; in the ftYd Stars 30' De la Hire and Caffini make it 32' in all the heavenly Bodies. Tycho, again, makes the ^sfraCtion of the Sua at 33 Altitude, to be 55"; but Cajjini i\ 45"

Fa. Laval in 1710, 22 Jan. obferv'd the meridian Altitude of the Sun to be 70° 25' 50"; and on the 23 of June, obferv'd the fame to be 70 ° 2.6 1 o'', which is io"more, that iliould be lefs. — Having met with fbme like Observations before, he takes occafion to fufpect the Refraction to be varied according to the different Winds which blow from the different Quarters. — When the North-Weft Wind blows, he thinks the Refraction is the great- eft; and adds, from Obfervations made at St. Baume, and St. Pi- Ion, that the Refraction at 24 Fathoms above the Surface of the Sea, is double that at 600 Fathoms. See Horizon. — Huygcns long ago obferv'd the Refraction to be changed every Hour ; tho' his Experiments were made at very little Altitudes, and in Ter- reftrial Objects.

The Refraction diminimes the right and oblique Afcenfions of a Star 3 and increafes the Defcenfions: It increafes the Northern Declination; diminifhes the Southern. See Ascension, De-

SCENSION, <&C.

Refraction, in the Eaftern Part of the Heavens, diminifhes the Longitude of a Star ; but increafes the fame in the Weftern Part ot the Heavens : It diminifhes the Southern Latitude, and in- creafes the Northern. See Longitude and Latitude.

The Refraction therefore, is by no means to be over-look'd in Aftronomy. — 'Tis absolutely neceflary to the determining of the Phenomena of the heavenly Motions, to a Degree of Accu- racy; Co that the antient Aftronomy, where no regard was had to it, muft of neceffity have been exceedingly defective on this very account. See Astronomy.

To obferie the Refraction of a Star, &c.

i°. Obferve the meridian Altitude of a Star near the Zenith; whence the Latitude of the Place being known, the true Decli- nation of the Star is eafily had, the Star being now void of any feniible Refaction. See Declination.

i°. Obiervcthe Altitude of rhefamcStar in any other Degree, and note the Time by a Pendulum. 3 . For the given Time of Oblervauon, from the Declination of the Star, compute its true Altitude. See Altitude.

This being thus found lefs than the Altitude obferv'd ; fubftract the one from the other; the Remainder is the Refraction ior that Moment, i n that Degree.

Refraction of Altitude, is an Arch of a vertical Circle, as §/> ( Ta 'f- Agronomy, Fig. 28.) whereby the Altitude of a Star SE, is increas'd by the RefraCl'un. See Altitude.

Refraction of Declination, is an Arch of a Circle of Decli- nation, as /I, whereby the Declination of a Scar/ DS isincreas'd ordiminiiVdhy the Refraction. See Declination.

Refraction of Afcmfon and Defcenfion, is an Arch of the Equator, Dd, whereby the Afcenfion and Defcenfion of a Star, whether right or oblique, is increafed or diminifhed; by means or' the RejraCtioji. See Ascension, &c.

Refraction of Longitude, is an Arch of the Ecliptic, Tt, whereby the Longitude of a Star is increas'd or diminifhed by means of the Refraction. See Longitude, &c.

Refraction of Latitude, is an Arch of a Circle of Latitude / 1, whereby the Latitude of a Star, Tf, is increafed or dimi- nifhed by means of Rcfradio?i. See Latitude.

REFRANGIBILITY of Light, the Difpofition of the Rays to be refracted. See Refraction.

A greater or left Refrangibility is a Difpofition to be more or lefs retracted, in paifmg at equal Angles of Incidence, into the fame Medium. See Light, Medium, &c.j

That the Rays of Light are differently refrangible-, is the Foun- dation of Sir ifaac Newtorfs whole Theory of Li^ht and Co- lours.— The Truth of the Principle will appear from the follow- ing Experiments.

i p. A Ray of Light being received through a little round Hole into a dark Room, upon aGlafsPrifm, ABC, {Tab. Optid-s. Fig. 55.) in fuch manner as to pafs through it near the Angle C ; the various Colours of the Pvainbow will be feen painted 10 all their Splen- dor on a white Paper, EF; viz. the Red in E, then the Yel- low, then Green, Blue, and at laft Purple, or Violet; and on •whatever Body you receive the Light, ftill the Colours will be the fame.

Yet this colour'd Light is ftill propagated, like other Light, in right Lines; 'tis reflected, too, like other Light, from a Mirror, and refracted through a Lens ; yet retains its Colours both after Refraction and Reflection. — When collected into a Focus, the Rays degenerate into a very bright White; but upon diverging again from the Focus, refume their former Colours.

Hence, i°. Since' nothing here happens to the Rays in paffing the Prifm, but that they are refracted, both in entering, and ia quitting it : (See Prism. ) Light is converted into thofe Co- lours by mere Refra&ion.

2. 9 . Since the colour'd Rays are ftill propagated in right Lines, if reflected from Mirrors, or refracted in Lens's; they ftill re- tain all the Properties of Light, and therefore are ftill Light.

3 . Since the feveral colour d Rays decuflated and mix'd to-^ gether in the Focus, appear White ; but after Separation, beyond the Focus, recover their former Colours; therefore Red, Yel- low, Green, Blue, and Purple Rays mix'd together in a conve- nient Proportion, conftitute a Refplendent white. See White.

Note, The Experiment will fucceed if the Room be not dark, only the Colours will be the lefs vivid.

2. A Prifm DEF (Fig. 66.) being fo difpofed as that the Re- fractions of the Rays both at their Entrance and Exit, are equal ; (which is obtain'd, by turning it flowly round its Axis, till the colour'd Light, which now riles, and now falls, appear ftationary between the two.) In the middle Space between the Prifm and the colour'd Light painted on the Wall, place another Prifm GH to receive the colour'd Light LM. After a iccond Re- fraction in this fecond Prifm the colour'd Light painted on the Wall, IK, will be inclined to a like Light NO, feen there, even when the Prifm GH is removed; fo as the blue Extremities N and I will be further a-part than the red ones K and O.

Hence, i°. The blue Rays muft of Neceftity be more re- fracted than the red ones; and there is, like^ife, an unequal Re- fraction in the intermediate Rays.

Hence, therefore, the Sun's Rays are not all of the fame Re- frangibility, confequently, not of the iame Nature. See Ray.

3. Thofe Rays are moft Refrangible, which are moft Reflexi- ble. See this proved under Reflexieility.

The Difference between Refrav.gi'mlity and Reflexibility was firft difcover'd and publifh'd by Sir Ifaac Newton in 1675, in the Phil. Tranf.ci. and from that time vindicated by him, from the Objections ot feveral Authors ; particularly F. Pardies-, M. Ma- riotte, Fr. Linus, or Lin, and other Gentlemen of the Englijb College at Liege : At length it was more fully laid down, illuftra- ted, and confirmM by great variety of Experiments in his immor- tal Opticks.

But, further, as not only thofe Colours of Light produced by Refraction in a Prifm, but alfo thofe reflected from Opake Bo- dies have their different Degrees of Pvefrangibility and Reflexibili- ty; and as a white Light aiifcs from a Mixture of the feveral co- lour'd Rays; the fame great Author concluded all homogeneous_ Light to- have its proper Colour, correiponding to its Degree of Refrangibility, and not capable of being chang'd by any Reflexi- ons, or any Refractions ; "that the Sun's Light is compofed of all the primary Colours ; that all compound Colours arife from a mixture of the primary ones, &c. See Colour.

The different Degrees of Refrangibility, he conjectures to arife

from the different Magnitude of the Particles whereof the dirfe-

11 R rent