INFUSORIAL ANIMALCULES. 43 



myself determined on presenting the public with detailed 

 methods of constructing and testing achromatic micro- 

 scopes. I further directed the attention of my workmen to 

 the subject, and gave them, from time to time, such infor- 

 mation, as, with their skill and perseverance, might advance 

 them in this branch of art, and I believe, up to the present 

 time, the only successful artists in this country are those 

 who have been in my employ. 



In cases where an achromatic microscope cannot be 

 procured, recourse should be had to single lenses or 

 doublets, for the ordinary compound, however well con- 

 structed, cannot be depended upon. 



With respect to the cost of an effective microscope, 

 with a moderate equipment of apparatus, such for example 

 as the one described in the 6th chap, of the Microscopic 

 Illustrations, which is of the best construction I have seen, 

 the price would now be from 20 to 30 guineas. As there 

 are a great many persons who require only a plain, sound 

 instrument, of more moderate cost, I have deemed it ex- 

 pedient to take this also into consideration, and, after 

 much application and repeated experiments, have at length 

 produced one, in every way suitable to the case. Such is 

 my vertical tripod achromatic microscope. It may be 

 stated that nine-tenths of the observations recorded in this 

 work may be repeated and tested by this microscope. 

 On comparing the above instruments with that used by 

 Dr. Ehrenberg, there is no doubt that, in point of mecha- 

 nical construction, they are greatly superior, whilst the 

 optical part is equal to any with which his researches have 

 been made. 



In adverting to this portion of my subject, I am 



