CORRESPONDENCE. 243 



power between themselves, which is, I am sorry to say, not miknown 

 amongst our own makers. This never tends to inspire confidence in 

 the accuracy of the rest of the work, and that it is by no means un- 

 avoidable is proved by the closeness of Messrs. Powell and Lealand's 

 working. These gentlemen made me a special 1 inch of 20° and 

 ^ inch of 40°, and I found the power to be (with their No. 1 ocular) 

 X 52 and x 105 respectively ; their catalogue giving the round 

 numbers x 50 and x 100. 



Undue discrepancies in amplifying power may cause confusion in 

 purchasing lenses. I may name a case in point. Through the kind- 

 ness of Mr. Wenham I became the fortunate possessor of the original 

 ^th which he had worked uj) to 120° on his new principle. The ex- 

 quisite defining and resolving power of this particular glass rendered it 

 a most valuable acquisition, but it fell short in one respect, I 

 imagined that in it I was adding to my series a power a step higher 

 than my Andrew Eoss :jth (1854), but I found it, in fact, very much 

 lower. With Eoss' 13 ocular at 10 inches, and with the screw- collars 

 set half-way, the -|th gave x 406, and the |th x 360 only. This could 

 not have been surmised from any published list, since A. Eoss's 

 catalogue of 1853 gives his ^th with B = x 350, while Eoss and 

 Co, in 1872 set down their ith as x 425 and in 1875 as x 400. 



While on the subject of amplification I may be excused for alluding 

 to a point which I have reason to know is occasionally misappre- 

 hended, however trite it may appear to the bulk of your readers. The 

 published lists of magnifying power in the opticians' catalogues must 

 not be supposed to furnish even a rough approximation to the amplifi- 

 cation with which an object will be seen in the microscope with those 

 objectives. To admit of comparison, these lists are compiled on the 

 conventional assumption that the distance from the eye-lens to the 

 stage is always uniformly 10 inches ; whereas with low powers and 

 long bodies it may really reach 16 or 18 inches, with a proportionate 

 increase in magnifying power. Hence every microscopist should work 

 out for himself two tables of the power of each of his objectives with 

 his various oculars; one at 10 inches to compare with published lists, 

 and the other, and by far the more important one, at the loorking distance 

 in each case. It is almost superfluous to hint how this is done. The 

 microscope being set horizontally, the paper on which the images of 

 the stage-micrometer lines are to be marked, is placed in the first case 

 exactly 10 inches below the centre of the reflecting surface of the 

 camera, and in the second, exactly as far from it as that is from the 

 surface of the micrometer, when the images of the lines (or the out- 

 line of any object) will be traced of precisely the same size as they 

 would appear to the eye on looking through the instrument in ordinary 

 work. " Personal equation," however, comes into play here. The 

 magnifying power as determined by a long-sighted and a short-sighted 

 observer will vary by a very notable constant diflerence. In all such 

 trials the screw-collars (if any) should be set to the same point in 

 each case, or serious discrepancies may arise, perhaps not always 

 unintentionally. 



Such elaborate comparisons between different objectives as those 



