Ihe Lantern Microscope. By Lewis Wright. 199 



In the convergent cone of rays from the lantern condenser, is 

 placed a parallelizing plano-concave lens, giving an approximately 

 parallel beam (i. e. as nearly parallel as admitted by the conditions 

 stated above) of about If in. diameter. This lens is of highly 

 dispersive glass, and therefore to a large extent corrects the 

 chromatic effects of the lantern condenser. 



In the same position, nearly, is placed the alum trough. In 

 but one lantern Microscope which I have examined, is there any- 

 thing like adequate protection from heat for balsam slides. I 

 have found it advisable to employ a full inch of alum solution, and 

 in addition, to form the second side of the cell of a doiible plate of 

 glass, the two cemented together by Canada balsam. This layer 

 of balsam absorbs any special balsam-heating rays which get 

 through the alum. With these arrangements protection from heat 

 is not comparative, but absolute and perfect. Less than this is not 

 real protection ; for the heat in the conjugate focus of a good lime- 

 light is sufficient to ignite black paper. 1 may remark that I have 

 seen a layer of balsam employed to protect bakam slides years ago, 

 but am not now certain by whom. 



For ordinary purposes, I cement the concave lens with balsam 

 on the alum trough, thus making this lens the second of the two 

 glass plates. By this expedient the loss of light by two reflections 

 is avoided. 



From the parallelizing concave lens to the stage is about 

 5 inches. Less than this would suffice for mere focusing pur- 

 poses, with plain work only ; but this distance is not enough to 

 produce much of the waste by scattering above alluded to, while it 

 allows of a really good-sized polarizing prism being introduced 

 when necessary, such as will give a polarized slide-disk of 3/4 in. 

 diameter — none too much for rock sections. Also it appeared to me, 

 from such experiments as I could make, on the sole occasion I was 

 able to borrow for a few hours an " achromatic condenser," that 

 the latter gave more light and worked better when not placed in 

 parallel light, but just after allowing the rays to cross from the 

 lantern condenser, without parallelizing them at all. I therefore 

 provided for this, which is easily accomphshed by having a spare 

 alum-cell with a plane second side, instead of the concave lens. I 

 since find that the same conclusion as to using crossed rays has 

 been reached by Dr. Hayes,* though for somewhat different 

 reasons. At all events, it seemed desirable to provide the widest 

 possible range in the optical manipulation of achromatic condensers. 



The end of the condenser-focusing rack-tube, is a tube-fitting 

 of the standard l^-in. substage gauge. I adopted this in order 

 that any standard apparatus, such as a paraboloid, or achromatic 

 condenser, may be used with facility. As the tube racks out 



* See this Journal, iv. (1884) p. 805. 



