ON THE THEOaY OP THE MICfiOSCOPE. 219 



effect of the whole. And here, before all things, it becomes obvious 

 that the factors on which correctness of image in the centre of the 

 field, and consequently the maximum of good performance 

 depend, namely, chromatic and spherical aberration (in 

 the stricter sense, as before explained), pertain to the 

 functions of the objective alone, upon which no influence 

 of the eye-piece, however constructed, can produce any marked 

 effect. And further, that any errors in definition of the image 

 to which the ocular might in part contribute, do not diminish the 

 attainable perfection of the whole, except in so far as there remain 

 unavoidable residual aberrations due to the deficiency of focussing 

 function of the objective (in the special sense already described). 

 Gross faiilts of construction excepted, the ocular, however simple 

 its construction, may be considered practically free from error in 

 comparison with the anomalies of colour and amplification which 

 arise from faulty action of the objective in respect to its focussing 

 function. Hence it follows that the maximum efficiency of 

 performance depends entirely on the objective, and that no 

 imaginable improvement in the ocular can influence it in any 

 degree. Also that the conditions under which the ocular is made 

 to act — as, for instance, in obtaining greater amplification by 

 increased length of tube or strength of ocular — are entirely 

 indifferent so long as such arrangements are kept within practical 

 bounds ; and therefore the excellence of performance will not be 

 lessened provided the objective used be chosen to suit the assumed 

 conditions. Arguments advanced in England (Pigott) in favour 

 of a long tube, as recently in Prance, by Prasmowscki, in favour 

 of a short tube, are equally untenable in theory; and the 

 supposed differences of effect prove to have no real existence when 

 examined under conditions which are truly comparable. In like 

 manner the reports of extraordinary performance of this or that 

 ocular, supposed to be due to its special construction, prove, when 

 tested by theory and accurate experiment, to be illusory in so far 

 as any real increase of optical power is in question, and not 

 merely some advantage of secondary and only collateral 



