332 Acccmnt of Mr Pritchard's Smgle Lens Microscopes 



parison was made, Mr Pond has had two aplanatic micro- 

 scopes made for him, the one by Mr TuUey, and the other by 

 Mr Dollond, with double triple object-glasses, and both of 

 these fine instruments, executed by the first artists in Europe, 

 are equalled by the simple sapphire lens. 



Our late distinguished countryman Dr Wollaston likewise 

 compared the sapphire lens with these three instruments at 

 the Royal Observatory. He also was perfectly satisfied that 

 it was equal to these fine instruments ; and he immediately 

 ordered for his own use a set of the sapphire lenses. 



Now since these three instruments with which the simple 

 sapphire lens has been compared have no aberration of 

 refrangibility, the reflecting microscope having necessarily 

 none, and the other two having it completely corrected, while 

 the sapphire lens has the disadvantage of all its uncorrected 

 colour, and yet ecjuals them, it is manifest that the sapphire 

 lens must surpass them completely/, when it is put on the same 

 footing 171 point of refrangibility, that is, when the objects are 

 illuminated with homogeneous light. 



We would, therefore, strongly recommend it to Mr Pritchard, 

 to turn his attention to the simplest method of obtaining ho- 

 mogeneous light, and to have the apparatus made to accompany 

 his lenses. For opaque objects, coloured silks and paper produce 

 a very fine effect, and from the vegetable world the most per- 

 fect homogeneous light may be obtained. In another paper 

 we shall give an account of a series of experiments, which we 

 have made on this subject with the flowers and leaves of 

 plants in different stages of their growth ; but we may men- 

 tion, in the meantime, that the petals of the scarlet lychnis re- 

 flects at a certain stage of its growth a pure homogeneous red, 

 upon which opaque objects are most beautifully seen. 



There are two advantages of the diamond and sapphire lenses 

 which we must not omit to mention. From their great hard- 

 ness, they will never be scratched or injured by use like those 

 made of glass ; and from the same cause, the artist is enabled 

 to burnish them into small flat plates of brass, which prevents 

 the possibility of their being lost, and renders them capable of 

 being cleaned without danger. 



As Mr Pritchard is now executing for us one of his sap- 



