loo MTCKOSCOPES FOR 



of the scales of insects uud otlier similar objects as teds — nay, even 

 certain shells of BaciUaria, the most difficult of test objects (Plates 

 XIX, XX.) — we may see with wonder the different markings on the 

 siu'face of these dust-like atoms, but our admii-ation will be carried 

 still higher, by the development of those brilliant coloui's and delicate 

 tints which are discoverable in many sj)ecies of the minute Infusoria. 

 The criterion of a good microscope, then, will be, that not only the 

 fonns of these little creatiu-es, their curious structures, organization, 

 and digestive apparatus, are exhibited with perfect clearness, but that 

 there is also shown the deep and brilliant colouring of their visual 

 organs, and the delicate tints of their variable, reti'actile, and loco- 

 motive processes. 



The various methods of managing the microscope, and the different 

 apparatus subsidiary to it, have been so fiiUy expatiated upon by the 

 late Dr. Goring, my much esteemed colleague, and myself, in our 

 joint works, Microscopic Illustrations, Microscopic Cabinet, Micro- 

 graphia, 8fc., that it will only be necessary here to notice, briefly, a 

 few particulars, wliich more especially relate to the subject before 

 us, and to refer the reader to those works for fui'ther infonnation. 

 As the expense of instruments, in the commencement of oiu' studies, 

 is often an important consideration, a few words on this head maj- not 

 he considered inappropriate, on this occasion. Dr. Ehi-cnberg infonns 

 us, that he began his observations with a common microscope, and, 

 although by his superior talent and imwearicd laboiu", he ^vas enabled 

 to make some important discoveries, yet he delayed, for some years, 

 the publication of them, imtil he could verify them with better 

 instruments. 



At the period when our fii-st publication was announced (1827) an 

 interest in microscopic science had to be created, to which I may add, 

 that the achi-omatic microscope was then in its infancy. Dr. Goring 

 having only a short time previous (1824) discovered the conditions on 

 which their efficacy depended, namely, large angular aperture free from 

 aberration. That publication aroused the attention of sjieutilic men 

 to the subject, but instruments, even such as those then made, were 

 veiy difficult to procui-e. To obviate this difficulty. Dr. Goiing and 

 myself determined on presenting the public with detailed methods of 

 constructing and testing achi'omatic microscopes. I fui'ther dii'ccted 



