IT need not be said that in astronomical observation it is always desirable, to say the least of it, to have a tolerably correct estimate of the magnifying power actually in use. This has hitherto been only attainable either by means of the maker's valuation, or through the employment of the apparatus unfortunately termed a “dynameter,” a word which every classical scholar would wish to see as soon as possible dismissed from circulation. The former alternative is, I am sorry to say, often far from reliable; the latter involves an outlay not within the reach of every astronomical student. The Rev. E. L. Berthon, Vicar of Romsey, Hants, well known already for many ingenious and valuable inventions, has recently devised a little apparatus for attaining the same object, which deserves high commendation. Its very moderate price places it within the reach of all; and its accuracy appears equal to that of instruments of more complicated construction and higher pretension. I have heard on excellent authority that very little dependence can be placed on the estimates of magnifying powers too frequently furnished to purchasers. Eyepieces are both constructed and rated too frequently by “rule of thumb,” and their real, if measured, will be found widely different from their nominal power. Some opticians, as, for instance, the celebrated reflector-maker Short, have had an unfortunate reputation for exaggerating the power of their instruments, and without any suspicion of misrepresentation: such has been the case even at the celebrated Optical Institute of Munich, as appears by the corrections made by W. Struve in the numerical values of the Dorpat oculars, 94, 140, 214, 320, 480, 600, 800, 1,000, 1,500, 2,000, being respectively lowered by him on trial to 86, 133, 198, 254, 420, 532, 682, 848, 1,150, 1,500. In this instance, it is possible that some different mode of measurement may have led to the discrepancy. Uncertainty, it may be suspected, occasionally arises from this cause. I once undertook, at the special request of a friend, to verify with a double-image dynameter the power of some oculars constructed by a very eminent optician, whose name was an abundant guarantee for his good faith; but the. results, on which I bestowed a great deal of care and trouble, trusting only to averages of many repeated measures, did not agree satisfactorily with the maker's statement. I do not know whether it may have been generally noticed, but the remark is a very obvious one, that the limit of numerical error increases with the power, so that in the case at any rate of ordinary dynameters, minute accuracy in the estimates of very deep oculars bears evidence of its own futility. If it represents anything of value, it can only be the care and attention with which a mean was deduced from repeated trials; but even this would be better expressed in round numbers, as less likely to convey an erroneous impression to the uninitiated.