1905.] on High Power Microscopy. 15 



weeks ago has dejirived the world of the labours of one of the most 

 successful makers of these appliances, and one from whose inventive 

 powers appHed to this subject matter much might still have been 

 expected had his life been prolonged. But, while putting aside the 

 more technical and intricate questions related to lens construction, 

 which are and always will be connected with the name of Abbe, 

 attention may be drawn, and perhaps usefully, to a proposition con- 

 cerning the Hmit of useful magnifying power in the microscope 

 objective, which was formulated and proved by Helmholtz so long 

 ago as 1874, but has been strangely overlooked by writers on the 

 subject since that date. According to that theorem, the object glass 

 reaches the limit of its useful development in the direction of 

 increased magnifying power so soon as, by reason of the shortening 

 of its focal length, the diameter of the object glass in its principal 

 plane is reduced to something not much less than the diameter of 

 the pupil of the observer's eye. The reason of this may be shortly 

 stated in this way : the human eye is not a homogeneous and per- 

 fectly transparent body like a well made lens of glass or crystal ; on 

 the contrary, its mass is intersected by connective tissue, and very 

 commonly the aqueous humour is infested by minute opacities, 

 while through the whole structure blood corpuscles circulate, 

 carrying oxygen and minute shadows into every part. When the 

 retinal picture is formed of broad beams of light, these small 

 obstructions are unnoticed, the eye can look round them by means 

 of the unobstructed rays which enter into the beam. But if the 

 diameter of the beams which furnish the pictm-e is cut down to 

 something commensm-able with the diameter of the obstniction, 

 then the loss of hght occasioned by it is serious, and may, when 

 sufficiently pronounced, cause a visible shadow of the obstruction to 

 be projected upon the picture. The same effect of intrusive shadows 

 is produced by specks of dust and imperfections of polish in the 

 ocular, and by the combined effect of all these causes a practical 

 limit is put to the magnifying power which can be usefully employed. 

 Object glasses can be made, and have been made, with as short focal 

 length as -^^ of an inch. But they are mere curiosities, possessing 

 no practical advantage over the -1^, y^ ^^^ iV i^i common use. Added 

 magnifying power to any required extent can be obtained by means 

 of high-power oculars, and Helmholtz has shown that the image 

 formed in that way may be just as perfect as the image formed by an 

 object glass of higher magnifying power backed by a lower eye-piece. 

 In fact, the one system is the exact optical equivalent of the other, 

 and the object glass of greater focal length has the advantage of a 

 greater working distance from its object. Thus, in practice, the 

 high-power objective has come to have a diameter about equal to 

 that of the pupil of the human eye, but this — although a theoretical 

 reason for the rule was published thirty years ago, and by no less a 

 writer than Helmholtz — has in fact been reached as the result of 



