Theory of Illuminating ApiMvatus. By Dr. H. E. Frij^p. 505 



and concave mirror, are either omitted altogether or treated in a 

 manner which is at variance with the first principles of optical 

 science. The same erroneous teaching is even chargeable against 

 some of the diagrams illustrating refraction and reflection of light, 

 while the general poverty of diagrams explanatory of optical 

 problems is scarcely redeemed by figures of lamps and instruments 

 interesting only to the maker and mechanical draughtsman. So 

 again, when practical directions are substituted for theory, inconsis- 

 tencies which crop up while consulting " authorities " demonstrate 

 the present uncertainty of rule and inconstancy of result. On the 

 part of the general body of microscopists, this leaning towards 

 authority weakens the power of and trust in personal observation, 

 and proportionally lengthens the reign of divided doctrine and 

 opinion. Thus it happens that the first difficulty encountered by 

 the beginner — namely, the fitting illumination of objects viewed 

 through the Microscope — still remains the last difficulty to be over- 

 come by the experienced microscopist. 



B. Since then we look in vain for any systematic exposition of 

 the principles of action upon which the performance of the several 

 classes of illuminator (reflecting, refracting, or combined apparatus) 

 depends, it seems expedient to collect such evidence as our micro- 

 graphy may afford of the current doctrine and practice in England, 

 in order that we may form a just estimate of the subject before 

 attempting any alternative theoretical views. The extracts here 

 quoted are chosen solely on account of the optical principles, 

 implied though not always expressed, on which the practical 

 remarks are based. In the present paper the consideration of the 

 theory and practice of illuminating opaque objects is omitted, as 

 this part of the subject is more conveniently treated separately, 

 though not involving any essential difference of principle. 



Mr. Eoss pertinently remarks, " The manner in which an 

 object is lighted is second in importance only to the excellence of 

 the glass through which it is seen." And again, " The principal 

 question in regard to illumination is the magnitude of the illu- 

 minating pencil, particularly in reference to transparent objects. 

 Generally speaking, the illuminating pencil should be as large as 

 can be received by the lens, and no larger. Any light beyond this 

 produces indistinctness and glare. The superfluous light from the 

 mirror can be cut off by a screen having various sized apertures 

 placed below the stage" (diaphragm). 



Mr. J. Smith says, " In viewing transparent objects the plane 

 mirror is most suitable for bright daylight ; the concave for a lamp 

 or candle, which should have the bull's-eje lens, when that is used, 

 so close to it that the rays may fall nearly parallel on the mirror. 

 If the bull's-eye lens is not used the light-source should not be 

 more tlian five or six inches from the mirror. This latter is seldom 



