D I A 



727 



D I A 



meter. 



Diapason (or 11th,) which see. M. Capella supposed this inter- 

 . II val to be equal to eight tones and a semitone, but which 



no - is not correctly so, for 11th + 22 = 8T + J, 11th 

 , +22 = 7T+t + §, 11th + 2 = 6T+2t+S, and 

 1 lth + 2 = 5T + 3t + 3. M. Capella also says, that 

 the 11th is equal to 17 semitones and 34 dieses. See 

 Dieses lesser, of M. Capella. 



DIAPASON Ditone, an interval whose ratio is i, 

 = 8902 + l6f + 70m; the Tenth Major, (or Xth,) 

 which see. 



DIAPASON Imperfect, an interval whose ratio is 

 |-|, = 5762 + 1 If -f- 50m ; the Eighth Acute Minor, 

 (or 8th of Liston,) which see. 



DIAPASON, Semiditone, an interval whose ratio 

 is T ? T , = 7732 + 15f + 67m; the Tenth Minor, (or 

 10th,) which see^ 



DIAPASON, Superfluous, an interval whose ratio 

 is |f, =6482 -f- 13f + 56m; the Eighth Superflu- 

 ous, which see. 



DIAPASON Stops, in Music, are ranges of pipes 

 through the scale of an organ, which are considered as 

 the standard of pitch, and are usually first tuned ; after 

 the pitch of C, in the middle octave, answering to the 

 tenor cliff or ledger-line above the bass or below the 

 treble staves, has been adjusted, by means of a standard 

 pipe, or by some of the methods described under our 

 articleCoNCERT Pitch. The diapasons are of two kinds; 

 the open diapason stop, which is a range of cylindrical 

 pewter pipes, some of which are commonly gilt, and 

 exhibited in the front of the instrument ; and the stopt 

 diapason stop, a range of square wooden pipes, with 

 plugs or stoppers in their upper ends, which are drawn 

 out or shoved in, to effect the tuning. In order to 

 yield the same notes, these stopt wooden pipes are only 

 about half the length or height of the open metal ones. 

 See Organ, (g) 



DIAPENSIA, a genus of plants of the class Pentan- 

 dria, and order Monogynia. See Botany, p. 140. 



DIAPENTE, in Music, or Pentachord, is an interval 

 whose ratio is •§-, —358 2 -f 7f+31m; the Fifth 

 Major, (or Vth,) which see. (^ ) 



DIAPHANOMETER, from l,*tpxnm, transparency, 

 and ftirpov, a measure, is the name of an instrument in- 

 vented by Saussure, for measuring the transparency of 

 a portion of the atmosphere. 



The Cyanometer, which we have already described 

 under its proper head, by ascertaining the intensity of 

 the blue colour of the sky, enables us to measure the 

 total effect of the vapour and evaporation diffused 

 through the whole depth of the atmosphere ; whereas 

 the Diaphanometer, by measuring the transparency of a 

 portion of the atmosphere of limited extent, is intended 

 to shew the quantity of vapour or evaporation existing in 

 that portion. 



The distances at which the same object ceases to be 

 visible in different states of the atmosphere, are obvi- 

 • ously relative measures of the transparencies of the por- 

 tion of the atmosphere, between the object and the ob- 

 server, at the times when the observations were made ; 

 and hence it was Saussure's first difficulty to find 

 objects, the disappearance of which could, at a certain 

 distance, be ascertained with the utmost accuracy. He 

 found that the extent of disappearance could be more 

 accurately perceived when a black object was placed 

 upon a white ground, than when a white object was 

 placed upon a black ground ; that the results were still 

 more precise, when the disappearance was observed in 

 sunshine, than when it was observed in the shade ; and 

 that they were still more correct, when the white shade, 



surrounding a black circle, was itself encircled by a Diaphano. 

 ground of a darker hue, me t er. ^ 



The following account of Saussure's experiments, by y 



Dr F. W. Murhard, of Gottingen, is so short and per- 

 spicuous, that it will not admit of abridgment. 



" If a circle totally black, of about two lines in dia- Saussure's 

 meter, be fastened on the middle of a large sheet of pa- Diaphano. 

 per or pasteboard, and if this paper or pasteboard be meter * 

 placed in such a manner as to be exposed fully to the 

 light or the sun, if you then approach it at the distance 

 of three or four feet, and afterwards gradually recede 

 from it, keeping your eye constantly directed towards 

 the black circle, it will appear always to decrease in 

 size the farther you retire from it, and at the distance 

 of 33 or 34 feet will have the appearance of a point. 

 If you continue still to recede, you will see it again en- 

 large itself; and it will seem to form a kind of cloud, 

 the darkness of which decreases more and more, ac- 

 cording as the circumference becomes enlarged. The 

 cloud will appear still to increase in size, the farther 

 you remove from it; but at length it will totally dis- 

 appear. The moment of the disappearance, however, 

 cannot be accurately ascertained ; and the more expe- 

 riments were repeated, the more were the results dif- 

 ferent. This is an observation perfectly accurate ; and 

 having myself made a series of experiments under like 

 circumstances, I am the more convinced of the truth 

 of it. 



M. de Saussure having reflected for a long time on 

 the means of remedying this inconveniency, saw clear- 

 ly, that, as long as this cloud took place, no accuracy 

 could be obtained ; and he discovered that it appeared 

 in consequence of the contrast formed by the white 

 parts, which were at the greatest distance from the 

 black circle. He thence concluded, that if the ground 

 was left white near this circle, and the parts of the 

 pasteboard at the greatest distance from it were covered 

 with a dark colour, the cloud would no longer be vi- 

 sible, or at least would almost totally disappear. 



This conjecture was confirmed by experiment. 

 M. de Saussure left a white space round the black 

 circle, equal in breadth to its diameter, by placing a 

 circle of black paper, a line in diameter, on the middle 

 of a white circle three lines in diameter, so that the 

 black circle was only surrounded by a white ring, a 

 line in breadth. The whole was pasted upon a green 

 ground. A green colour was chosen, because it was 

 dark enough to make the cloud disappear, and the easiest 

 to be procured. 



The black circle, surrounded in this manner, with 

 white on a green ground, disappeared at a much less 

 distance, than when it was on a white ground of a large 

 size. 



If a perfectly black circle, aline in diameter, be past- 

 ed on the middle of a white ground exposed to the 

 open light, I can observe it at the distance of from 44 

 to 45 feet ; but if this circle be surrounded by a white . 

 ring, a line in breadth, while the rest of the ground is 

 green, I lose sight of it at the distance of only 15^ feet. 



According to these" principles, M. de Saussure deli- 

 neated several black circles, the diameters of which in- 

 creased in a geometrical progression, the exponent of 

 which was 4-- His smallest circle was f or 0/2 of a line 

 in diameter; the second, 0.3; the third, 0.45; and so 

 on to the sixteenth, which was 87-527, or about 7 inches 

 3| lines. Each of these circles was surrounded by a 

 white ring, the breadth of which was equal to the dia- 

 meter of the circle, and the whole was pasted on a 

 green ground. 



