1905.] on High Power Microscopy. 19 



From this principle it follows that with very high magnifying 

 powers the emergent pencil of light will become extremely small. 

 Thus, in a microscope exhibiting to the eye a magnification of what is 

 conventionally accounted 1000 diameters, the pencil of light which 

 enters the eye has a diameter of about yJo inch. The actual 

 magnifying power of an instrument reckoned at 1000 according to 

 the opticians' convention is about 80. It will, therefore, be under- 

 stood that if photographs having any considerable magnification 

 are to be produced it must be by the use of extremely narrow beams 

 of Hght. 



Reference has been already made to the inconveniences which 

 beset the employment of such very narrow beams of light. They are 

 more serious even in visual microscopy than in photomicrography, for, 

 although it is possible by careful polishing and scrupulous cleaning 

 to purge a lens of dust and obstructions, it is not possible by any 

 expedients so to clear the eye. Eyelashes, tears, and musccz iwUtantes 

 at least will be introduced into the picture in addition to any specks 

 which may be lodged on the ocular or on the back of the objective. 

 Hence a limit is soon put to the enterprise of the instrument maker 

 who essays to exhibit a really large scale image to the eye of his 

 customer. Fig. 7 is a photograph of typhoid bacillus blemished in 

 this way. The photograph does not, of course, exhibit eyelashes or 

 specks seated in the eye. Blemishes of that sort would in practice be 

 added to what is here shown, with the result of still further deterior- 

 ating the image. This particular specimen has been produced under 

 extreme magnifying power, such indeed as is never used for direct 

 vision, and is used only in connection with photography when special 

 appliances are employed to avoid the use of an ocular. This, however, 

 has been taken with an ordinary ocular and by means of a camera 

 specially constructed to fill the exact position taken in the optical 

 system of the microscope by an observer's eye. As it stands, the 

 magnification is about 7000 diameters, the picture having been 

 enlarged to a convenient scale for reproduction by photographic pro- 

 cess. The original negative had a magnification of 1900 diameters, 

 equivalent to about 6000 according to the opticians' convention, for 

 the camera used has about one third of the magnifying power of the 

 conventional human eye. It is not surprising that microscope makers 

 have given up the attempt to produce pictures upon this scale of 

 magnification, seeing that the enlargement of scale involves so much 

 corruption of the image. 



Down to the present time this defect of a highly magnified image 

 has been thought to be manifestly insuperable. Even if the users of 

 microscopes could be relied upon to take the extraordinary pains 

 necessary to keep their lenses absolutely clean, they could not keep 

 their eyes clean. Even to preserve the ocular against dust is no 

 small matter. It might be supposed that the ocular which supphed 

 the dust and minute hairs that so seriously impair Fig. 7 was selected 



c 2 



