36 iiEPoiiT — 1876. 



Light and Heat. 



Phvlometnc Measurements of the Mar/neto-electric Light. 

 By Captain Abney, F.B.S. 



Determination of the Conductivity of Heat by Water. By J. T. Boxtomley. 



Oyi the Testings of Large Objectives. 

 By Howard Grtjbb, Master of Engineering, Trinity College, Dublin. 



In the testing of large objectives, wlien the corrections have been made to be very 

 nearly perfect, a difficulty is sometimes felt in determining what, if any, corrections 

 remain desirable, and also of determining in a simple way the anioimt of the desired 

 alteration. 



For the chromatic aberration one plan often pursued by the optician is to slightly 

 overcorrect the objective in the iirst instance, and then to separate the crown and 

 flint (thus reducing the correction) until the best result is attained, when the 

 amount of separation required becomes by a simple calculation a measure of the 

 necessary alteration in the curves. 



This is an extremely useful practical an-angement ; but unfortunately it is appli- 

 cable to onlv one of the four possible errors, besides being troublesome and some- 

 what dangerous in the case of large objectives. 



The desirability of some simple plan of introducing, pro tempore, a small + or 

 — effect of chromatic or spherical aberration, and of being able to accin-ately esti- 

 mate the quantity of such, has been very apparent to me on seA'eral occasions ; for 

 I have frequently found the best judges of such matters differ in their estimate of 

 final correction, "and unable to agree thereon; and I have also often found a diffi- 

 culty in satisfying myself that the best balance of corrections had been attained ; 

 whereas if it "were possible to introduce a small amount, pro tempore, of + or — 

 correction, T could at once have perceived when I had overshot the mark. In 

 fact the perfection of any connection in an objective means the best balance bc- 

 ticeen two opposinff aberrations, and (just as in all cases of ascertaining balances) 

 it is difficult to determine the neutral point unless there be the power of trying on 

 both sides. 



To effect this, in preparing for the trial of the great objective for the new Obser- 

 vatory at Vienna (of 27 inches aperture), I am constructing four lenses or combina- 

 tion of lenses capable of being mounted between the objective and ocular, and with 

 a considerable range of motion in the axis of the telescope : — 



A. While it effects no other correction introduces a small amount of -(- 



chromatic aberration. 



B. Similarly introduces a small amount of — chromatic aberration. 

 0. „ ' „ + spherical aberration. 



J). „ „ — spherical aberration. 



The .'imount of any aberration introduced can be regulated by the position of the 

 coi'recting lens in the pencil of rays. 



Now, knowing the construction of these combinations and their position in the 

 pencil of rays from objective to ocular, the corresponding correction in the objective 

 IS an easily calculable quantity. Quite apart from the use to the optician I believe 

 the comfort of these appliances will be much appreciated by those appointed as 

 judges, particularly where, as in the case of the Vienna telescope, the testing of the 

 objective forms part of the work of a Committee composed of a considerable number 

 of Members. 



I have already experimented in this direction sufficiently to convince myself of 

 the great value of this system so far as the correction for chromatic aberration is 



