Theory of Illuminaiing Apparatus. By Dr. H. E. Fripp. 515 



microscopic object only an infinitesimally small amount of light can 

 be directed in the axial line upon the object, and that diaphragm open- 

 ings having a larger diameter than the object could have no eflect 

 of excluding light, though we know the contrary to be the case. 

 And this consideration brings me to remark on the singular 

 absence of any scientific exposition of the respective functions of 

 the plane and concave mirror, and the function of the diaphragm 

 when employed with the mirror ; although these illuminators 

 are of fundamental import to the general theory of illumination, 

 while their constant employment renders the science of then' effec- 

 tual application a matter of prime and lasting necessity.* In 

 comparison with this extraordinary omission, the perpetually exer- 

 cised ingenuity in construction of new " condensers " and reflectors 

 of all kinds, and the much-favoured discussion of exceptional and 

 often useless methods of illumination, fail in real and general interest. 

 Another remarkable omission is the absence of any study of 

 the effect of the object which intervenes at the crossing point of 

 converging illumination and the diverging pencils of light emitted 

 from the object. The fact is too obvious to escape the notice of 

 any practised observer that mere incidence of light is not the 

 sole determining condition of perfect definition. A complete theory 

 of illumination requires some special study of the physical effect of 

 the constituent particles of the object on the rays incident upon 

 them, whether from below or above. The intervention of the 

 object must break the continuity of these rays (how otherwise 

 would delineating shadows exist ?). Opaque parts interrupt more 

 or less the passage of a large number of rays, while diffraction 

 causes the additional phenomena of points of light which initiate 

 a new series of light pencils whose appearance in the objective 

 image is necessary to its perfect delineation of detail. And 

 even where light passes through transparent parts of the object 

 it is possible, nay probable, that its transmission is not unaffected 

 by differences of homogeneity and specific gravity of the substance 

 of the object (which has some thickness, even in the finest 

 preparations). Such facts radically dispose of the received notion 

 that transmitted light passes without change of direction through 

 the object. The difierent specific density, for instance, of the fluids 

 in which a preparation is preserved cannot but possess some 

 influence, just as different specific density and thickness of a glass 

 cover produce an eflect for which correction of the objective has long 

 been ackuowledged to be necessary. In short, the principle of 

 homogeneous immersion applies equally to the operation of the 

 particular preservative menstrua used, which comprise such various 

 fluids as glycerine, spirit, turpentine, saline solutions, &c. It is not, 



* The latest editions of our Microscope manuals are as bare of information in 

 this respect as the earliest. 



