The continental artists generally snake one circle turn closely, but freely, within another, and nearly in the same plane, as we have seen was directed by Vernier. The reading is much more pleasant and exact in this way. Troughton objected to it, that if a particle of dust should get between the circles it would necessarily grind and tear the edges of the circles, leaving a muddy and ragged ditch between them. We do not know whether this objection is confirmed by experience. The English artists generally place their verniers on thin plates which move upon the divided circles. There is some chance of rubbing, and a certainty of wearing, if the verniers press on the circle ; and if they stand off from it they are awkward to read, with a chance of considerable error from parallax. The subdivision by the vernier seems to be preferred by the German artists in general to that by micrometer microscopes, which are in England universally applied to large meridian circles, and indeed to all considerable instruments where the fixing of the microscopes is not subjected to a varying effect of gravity. On the side of the verniers may be pleaded cheapness, and freedom from changes, such as those which the scale of a microscope suffers when the distance between the limb and the object-glass of the microscope, or the body of the microscope itself, from expansion or other cause, is altered. On the other hand, the micrometer micro scope certainly admits greater magnifying power, keeps the observer away from the instrument, can be fixed with greater firmness, and remains more steady. It is not easy to fix a vernier firmly without running the risk of affecting the motion of the circle. On the whole we prefer the micrometer microscope, although it must be admitted that the perfection which the continental artists give to the centering of their circles and verniers may well cause a difference of opinion.
For small instruments, and those which, like the declination circle of an equatorial, are placed under different strains in different positions, the vernier is indispensable.
There is difficulty very often in getting the proper light on the divisions. It is desirable that those of the vernier as well as those of the limb should appear sharp and black, and the divisions before and after that which Is nearest to coincidence should be scrutinised In order to estimate the decimal or fraction which is wanted fur perfect coincidence. A more perfect setting will generally be obtained by making the divisions before and after the coincident division equally discordant, than by attempting to get a perfect coincidence. The observer should be careful to view the divisions directly, and in the antra of the magnifier, or he will have en error arising from parallax which may be considerable.
The truth of a vernier In one respect, that of ita embracing a proper portion of the limb, may be tried in different parts of the limb. If the circle be very excentrie this may give a little trouble at first, and be confounded with lad division. In ordinary cases, however, if there are opposite verniers, and their extreme division.; sometimes overlap and sometimes fall short of the corresponding portion of the limb, the mean will be true although tho excentricity is sensible. The number of verniers, may be either two, three, or four, at equal distances. Two aro absolutely necessary to get rid of excentricity. and three or four will also nullify any error, original or superinduced, which gives the circle en elliptic for m. But it is not easy in all ossea to apply these readings conveniently, and the fatigue of many readings is scarcely recompensed by a little superior accuracy. at least in well-made and well-divided instruments which are carefully handled.