40 PEACTICAi TREATISE ON 



make or understand the instrument, I would refer them to 

 the paper itself, which is contained in the 121st volume of 

 the Philosophical Transactions. From this discovery of Mr. 

 Lister's, in 1829, we may fairly date the rise and continued 

 progress towards perfection of the achromatic compound 

 microscope in England, and all cultivators of natural science, 

 as weU as the makers of the instruments themselves, are 

 largely indebted to Mr. Lister for publishing to the world the 

 valuable results of those labours, which certainly have formed 

 the groundwork of the plan on which all our first-rate opti- 

 cians now work, for whose success he has always most 

 zealously exerted himself, even to the examination, from time 

 to time, of their wonderful productions ; and it is but common 

 justice here to state, that we have now in this metropolis 

 three most eminent manufacturers of the compound achromatic 

 microscope, viz., Messrs. PoweU, Koss, and Smith, whose 

 instruments are without equal in this or any other country. 

 On consulting the dates at which these opticians respectively 

 commenced the manufacture of achromatic object-glasses, we 

 find that as early as March, 1831, Mr. Andrew Ross had 

 completed for Mr. Wm. Valentine a dissecting microscope 

 on an entirely new plan, being provided with coarse and 

 fine adjustments, stage movements, and a WoUaston con- 

 denser. This instrument, first described in the forty-eighth 

 volume of the Transactions of the Society of Arts, will be 

 more fully mentioned in the chapter devoted to the simple 

 microscope; although generally employed for dissecting, it 

 was nevertheless made capable of receiving a compound body. 

 The first microscope of this kind made by Mr. Ross is now 

 in the possession of R. H. SoUy, Esq., for which, in 1832, 

 Mr. Ross was also employed to construct a triple object-glass, 

 he, previous to the year 1831, having made lenses of the 

 precious stones, and acquired a knowledge of achromatism by 

 being connected with Professor Barlow, during the con- 

 struction of his fluid object-glass, and also in the arrangement 

 of his formula for computing the radii of curvature of an 

 achromatic one. Since the period above mentioned, Mr. Ross 

 has been constantly and actively employed in bringing these 



