It didn't spin webs and looked more to Evelyn like a mutated cat, but the moment she recalled Kiera's fear, she also realized that the cat-like creature would easily pass as a large spider.
To obviate this difficulty Felice Fontana of Florence (Saggio del real gabinetto di fisica e di storia naturale, 1 755) first proposed the use of spider webs in micrometers,' but it was not till the attention of Troughton had been directed to the subject by Rittenhouse that the idea was carried into practice.'
Two spider webs are stretched across the forks, one (t) being cemented in a fine groove cut in the inner fork k, the other (s) in a similar groove cut in the outer fork 1.
One of the most essential points in a good micrometer is that all the webs shall be so nearly in the same plane as to be well in focus together under the highest powers used, and at the same time absolutely free from " fiddling."
The nozzles of small lamps are inserted in the tubes L, L, for illuminating the webs in a dark field; the light from these lamps is admitted through apertures in the strong hollow cylinder above mentioned (for illumination, see p. 385).
To avoid such error Dawes used double wires, not spider webs, placing the image of the star symmetrically between these wires, as in fig.
Other astronomers use the two distance-measuring webs, placed at a convenient distance apart, for position wires.
I, 2 and 3), used to bolt the head of one of the screws, and the instrument was provided with a slipping piece, giving motion to the micrometer by screws acting on two slides, one in right ascension, the other in declination, so that " either of the, webs can be placed upon either component of a double star with ease and certainty (Mem.
But in OS measures index error can be eliminated by bisecting both stars with the same web (or different webs of known interval fixed on the same frame), and not employing the fixed web at all.
The five fixed webs are attached to the table which is secured to the bottom of the box by the screws p. The three movable webs are attached to the projections XX on the frame aa.
It seems also to take a very clean V cut, as the webs can be laid in their furrows with an astonishing ease and precision.
These furrows have apparently been cut in situ with a very accurate engine; for not the slightest departure from parallelism can be detected in any of the movable webs relative to the fixed webs.
The varnish to fix the webs is applied, not on the surface T as is usual, but on a bevel for the purpose,' the position of the webs depending on their tension to keep them in their furrows.
The result is that no trace of " fiddling " exists, and the movable and fixed webs come sharply together in focus with the highest powers.
A convenient feature in Repsolds' micrometer that the webs are very near the inner surface of the top of the box, so that the eye is not brought inconveniently close to the plate when high powers are used.
Da and The micrometer box, and of course with it the whole system of spider webs, is moved by the screw s, whilst the measuring web is independently moved by the screw S.
The electric lamp a gives illumination of the webs in a dark field, nearly in the manner described for the Cape transit circle micrometer; the intensity of illumination is regulated by a carbon-resistance controlled by the screw b.
Micrometers used for subdividing the spaces on graduated circles and scales have, in general, only a single pair of cross-webs or parallel webs moved by a single screw.
The focal length of the objective and the distance between the optical centre of the lens and the webs are so arranged that images of the divisions are formed in the plane of the webs, and the pitch of the screw is such that one division of the scale corresponds with some whole number of revolutions of the screw.
In all modern reading micrometers the cross webs of fig.
15, on one of the scales, by moving the whole micrometer box by means of the screw s; the pair of webs, moved by the screw S, is then pointed upon an adjacent division on the other scale.
If the reading for coincidence of the movable with the fixed webs is known, we then obtain from the single reading of S the difference from coincidence of the divisions of the two scales.
13 r qn ui, c It is generally possible so to arrange the method of observation as to eliminate the effect of an error in " the reading for coincidence of the webs " from the results.
X 5 mm., the sides of this square being parallel spider webs 4" of arc apart; the size of the square is reckoned from centre to centre of these double webs.
Each of the two micrometer screws X and Y moves a system of six parallel webs, placed 4" of arc apart from each other.
Webs serve not only for pointing on stars to determine their coordinates (in manner afterwards described), but also for estimating the diameters of the star-images in terms of these 4" intervals.