Dr. Wollaston^s Microscopic Doublet, 261 



form an image, every part of the lens must necessarily co-operate 

 informiny ihe image. If there is any double refraction given 

 by any part of the stone, it must infallibly make itself felt, 

 which is not the case, as a good sapphire lens forms a better 

 image than a glass one. It is scarcely possible to miss seeing 

 a double image, where there really is one, if the lens is only 

 useii as a simple microscope to view a bat's hair, or some similar 

 object, having intercepted light strongly condensed upon it. The 

 best way to make a comparison, is to get a lens which really 

 has a double image, and just to see what a precious mess it 

 makes of every thing viewed with it as a mere magnifier ; (for, 

 used as an object-glass it can hardly be said to form an image 

 at all.) The following are the characteristics of a diamond 

 lens, having a triple refraction, used as a simple microscope. It 

 has no determinate focus at all, not one of the lined objects can 

 be seen with it ; a bat's hair, seen with direct hght, shews three 

 images, overlapping each other ; nor can the outline of any 

 object whatever be discriminated with it. 



Another circumstance has been supposed to indicate double 

 refraction in the sapphire, namely, overlapping images. Now, 

 all microscopes and engyscopes, of what nature or kind soever, 

 give overlapping images, either when out of adjustment, or 

 when oblique light is thrown through them, however rigid their 

 adjustment may be, as is well known to every optician. This 

 is rendered peculiarly sensible with the class of lined objects 

 seen by artificial light, a complete penumbra, or double image, 

 of which may be seen to cross the true picture, when the object 

 is put out of focus ; moreover, in all microscopes, &c., the 

 lines on the scales of butterflies' wings, &c., may be seen 

 double, when the object is put a very little within the focus. I 

 mention these circumstances, that inexperienced persons may 

 not be led astray by them into a belief of things that are not. 



Par. 6. '• The cost, however, of such a lens, in comparison with glass, as 

 well as the readiness with which any number and variety of the latter 

 kind can be procured, led me to consider what simple combinations of 

 them might, perhaps, equal the sapphire lens in performance, without 

 great cost, or diflRculty of construction ; and though both Mr. Herschel 

 and Professor Airy have recently applied their superior talents to the 

 analytical investigation of this subject, it seemed not impossible that the 



