[45] Mr. C. Tulley recommended the combination of two achromatics together from nearly the beginning of his son W.’s labours upon them, who rejected the idea, together with myself, as giving rise to too great a complication, _always supposing that triple glasses must be used_. I myself combined two triple achromatics together, for experiment sake, in a very early stage of our proceedings, but liked not the result, though effects were certainly produced by the composition, which could not be obtained from the best individual triple one. [46] The theory on which these object-glasses are constructed is contained in a paper of Euler’s, published at St. Petersburgh, in 1774. Messrs. Chevalier have caused it to be inserted entire in the “Bulletin de la Société d’Encouragement de Paris,” No. CCLIV., for Aug. 1825. [47] [Illustration] This is the theoretical view of the case, the practical one is different, as frequently happens. _Opaque objects_ are not affected at all by the eccentricity of the axis of the body: nor can I recollect that I ever felt any particular inconvenience from the motion of the optical part, even with transparent subjects, unless it was thrown _very much indeed_ out of the axis of the illuminating mirror. On the other hand, it is notorious that _living_ aquatic insects and animalcules are the most _popular_ and _entertaining_ objects which microscopes can show. These are, for the most part, abundantly restless; and if the stage on which they are placed has any motion, their natural unquietness is so much exasperated, that it becomes almost impossible to get a good observation of them at all. I once set to work at making some drawings of a variety of new and original objects of this class (which, I trust, will one day be published) with a microscope having all the requisite motions applied to its stage, and am confident that I had thrice the labour fairly appropriate to the execution of my task from this oversight. The very tremor produced by the transition of a carriage in the street, is frequently sufficient to unsettle live objects when disposed to be still and quiet, and put them in a fidget for a quarter of an hour. It is very unfortunate that the mountings of optical instruments are made in general by mere mechanics, who seldom or never observe with them, and consequently know not the exigencies which occur in practice. It is still more unfortunate, that in the science of fitting up microscopes, an ounce of a man’s own wit is worth about a ton of his neighbour’s. Was it not that I dislike to verify this adage myself, I should recommend the following motion to be applied _to the body_:—let the socket of the arm which carries it have a smooth rotatory motion on the head of the bar in the usual way, conjoined with another horizontal one produced by rackwork attached to the said socket. Let the pinion which belongs to the latter movement be made very strong, so that a