14 REPORT — 1855. 



suddenly changed two points from a heavy blow of the sea on the ship ; on that of 

 another ship where a similar change took place on occasion of a collision; on that of 



the (name not mentioned) where the steering compass suddenly changed several 



points, and produced an error in the ship's position within 24 to 30 hours, which, 

 measured on a track chart by the first officer, in his. Dr. Scoresby's, possession, was 

 very nearly of the extent of the breadth of Ireland ! These various compass-changes 

 were plainly in accordance with the theoretic principles formerly published by the 

 author, with the exception of the latter, as to which the requisite data for tracing 

 the probable causes had not been furnished. 



Of compass-changes from strokes of lightning (one of the cases also predicted). 

 Dr. Scoresby adduced the instances of the Bold Biicdeuch and another ship, where 

 the compass suddenly went wrong to the extent of several points. 



Having elucidated, rapidly, these various magnetic pheenomena and others be- 

 longing to ships built of iron, and having given a variety of examples of great or 

 considerable alterations in the compass-direction of ships proceeding into southern 

 latitudes, the author recalled attention to his plan of a compass aloft, as affording, 

 in the absence of azimuths or other guidance from celestial observations, a simple 

 and effective mode of ascertaining the direction of the ship's course, and so, by 

 comparison with the steering compass, knowing its errors and the proper correction 

 to be made. This plan, he observed, when properly carried out, and a table of 

 deviations, if requisite, obtained, he believed to be perfectly safe and reliable ; and 

 he had much satisfaction in being able to state that it had not only been extensively 

 adopted by some of our first firms interested in the building and property of iron 

 ships, but had received the particular sanction and commendation of Mr. Airy, 

 Astronomer Royal, and Lieut. Maury, U.S. Navy ; that is, as being recommended 

 by both these gentlemen for adoption for determining safe compass guidance, or the 

 correction of adjusted compasses whenever they might be found to be in error. 



On the Achromatism of a Double Object-glass. 

 5y Professor Stokes, ilf.^., D.C.L., Sec.R.S. 



The general theory of the mode of rendering an object-glass achromatic by com- 

 bining a flint-glass with a crown-glass lens, is well know-n. The achromatism is 

 never perfect, on account of the irrationality of dispersion. The defect thence 

 arising cannot possibly be obviated, except by altering the composition of the glass. 

 It seemed worthy of consideration whether much improvement might not be effected 

 in this direction ; but the problem which the author proposed for consideration was 

 only the following : — Given the kinds of glass to be employed, to find what ought to 

 be done so as to produce the best effect ; in other words, to determine the ratio of 

 the focal lengths which gives the nearest approach to perfect achromatism. Two 

 classes of methods may be employed for this purpose. In the one, compensations 

 are effected by trial on a small scale ; in the other, the refractive indices of each 

 kind of glass are determined for certain well-defined objects in the spectrum, such 

 for example as the principal fixed lines. The former has this disadvantage, that 

 compensations on a small scale do not furnish so delicate a test as the performance 

 of a large object-glass. The observation of refractive indices, on the other hand, 

 admits of great precision ; but it does not immediately appear what ought to be 

 done with the refractive indices when they are obtained. After alluding to the me- 

 thod proposed by Fraunhofer for combining the refractive indices, which, however, 

 as he himself remarked, did not lead to results in exact accordance with observation, 

 the author proposed the following as the condition of nearest approach to achro- 

 matism : — that the point of the spectrum for which the focal length of the com- 

 bination is a mmimum shall be situated at the brightest part, namely, at about 

 one-third of the interval D E from the fixed line D towards E. The refractive 

 index of the flint-glass may be regarded as a function of the refractive index of the 

 crown-glass, and may be expressed with sufficient accuracy by a series with three 

 terms only. The three arbitrary' constants may be determined by the values of three 

 refractive indices determined for each kind of glass. The result is as follows: — Let 

 /^i» M2' Ms be the refractive indices for the crown-glass ; /xj', fi,2, /j-s the same for the 

 flint-glass ; fx, fj! the refractive indices of the two glasses for any arbitrary ray ; m 



