20 Mr. John W. Gordon [Feb. 17, 



as an awful example of what an ocular may come to under exemplary 

 neglect, but in fact there is nothing at all exceptional in its condition. 

 At'' moderate magnifying powers these intrusive particles are quite 

 invisible, and the ocular in fact was in the condition in which an 

 ordinarily careful user would be accustomed to employ it. Nothing 

 but the scale is out of the common, and with the high magnifying 

 power employed the narrow beams which can be quenched by these 

 diminutive obstructions are associated by a mathematical law. It was 

 perfectly reasonable, therefore, for the instrument makers to regard 

 the presentation to the eye of a satisfactory "super-amplified" image 

 as an insoluble problem of physics. 



But the insoluble problem has quite recently been solved, and by 

 the simplest imaginable expedient. Everybody knows that when a 

 magic lantern picture is thrown upon a screen it becomes just as 

 visible as a real object, that is to say, the angle from which it can be 

 viewed becomes so widely extended that the picture is visible to an 

 entire audience when transmitted by a screen, although it would be 

 visible only by the one or two beholders who could stand approxi- 

 mately in the axis of collimation, if the picture were aerial. The 

 same expedient gets over all the difficulties of high power magnifica- 

 tion in a microscope, and is employed in an instrument which you 

 will be invited to examine in the library this evening. Fig. 8 is a 

 diagram of this instrument. The arm which is extended from the 

 tripod at its side into the tube of the microscope carries at its free 

 extremity a small screen of finely ground glass. This glass screen is 

 by it held in the image plane of the microscope, and receives the 

 image formed by the object. Its grain scatters the light of this 

 image over a wide angle, exactly as the magic lantern screen scatters 

 the light of a lantern picture, so that the Lagrange relation between 

 the angle of the beam and its magnifying power is broken down, and 

 the screened image can now be seen, as if it were a real object, under 

 any desired angle. Such an image may be subjected without impair- 

 ment to high magnifying power, and so the eye-lens of the ocular is 

 replaced by a compound microscope fitted with a lialf-inch objective 

 and an ocular magnifying eight times. This systeni of " eye-piecing " 

 yields, of course, an enormously super-amplified image, and it was 

 with this apparatus, Init minus the ground glass screen, that the 

 ])hotograph reproduced in Fig. 7 was taken. We shall now be able 

 to see, by mejins of a strictly comparable ])hotograph of the same 

 ol)ject taken with the screen, what is the optical advantage accruing 

 from its use. 



One very ol)vious optical disadvantage there will cleai'ly be unless 

 steps are taken to avoid it, that is to say, the grain of the screen will 

 itself be visilile in the picture since it is dis})layed in the focal plane. 

 Fi"". !) shows this result, and although the dust has disappeared, the 

 grain of the roughened glass screen is itself a nuich worse blemish 

 than any reasonable accumulation of dust and fiymarks on the lenses 



