586 Sir David Salomons [Feb. 26, 



while it enables, as a consequence, the objective to be slightly further 

 removed from the slide ; i.e. giving what is termed a greater working 

 distance. The objection to this method is that, even when these 

 plano-concave lenses are corrected, the result, though greatly improved, 

 is not perfect. The second way, which is a i)erfect one, is that of 

 introducing an eye-piece. In both these methods, that the best results 

 may be obtained, the objective is made to occupy a position not very 

 different from that which it would do if emj)loyed on the table 

 microscope. 



In the eye-piece method almost the exact conditions can be com- 

 plied with for which the objective was made. I propose, therefore, to 

 show the subjects by the eye-piece method. The only objectives which 

 will be used are : (1) Zeiss's 35 millimetre projection objective, with 

 a sub-stage condenser, 4 inches focal length, placed a considerable 

 distance from the slide ; (2) Newton's 1-inch projection objective, the 

 sub-stage condenser as in the first case ; and (3) Zeiss's ^-inch 

 achromatic objective, the sub-stage condenser being Professor Abbe's 

 three-lens condenser with the front lens removed. In all three cases 

 the eye-pieces used are Zeiss Huyghens No. 2 and No. 3. 



In each instance I will mention the magnification in diameters, as 

 well as the number of times when reckoned by area, for the apprecia- 

 tion of those who estimate by area ; and I will also give the size to 

 which a penny postage-stamp would be increased, supposing it to be 

 made of indiarubber, and stretchable to any extent in all directions. 

 In presenting these figures I do not pretend that they are absolutely 

 correct, but as they have been ascertained under conditions similar 

 to those now existing the errors will not be very great. 



In consequence of the field not being quite flat, and the sections 

 having a certain thickness, although extremely thin in most cases, the 

 whole of the object cannot be in focus upon the screen at the same 

 time. By shifting the focussing screw slightly all parts may be brought 

 into focus successively. So-called greater depth of focus is obtained 

 by using an increased working distance ; and for projection work over- 

 correction for flatness can alone give a sharp picture all over with 

 very considerable depth of focus ; the difficulty of over-correction 

 being that, unless extreme care is taken, certain forms of distortion 

 may be introduced. By stojjping down the objective greater flatness 

 of field may be secured, but at the expense of light. There is 

 thus a choice of difficulties, and the least one should be taken. 



Turning now to the polariscope. Polarized light teaches us a great 

 deal concerning the structure of matter ; it is also a means of con- 

 firming the uudulatory theory of light. This subject is so large that 

 no attempt can be made to give even a general idea of the field it 

 covers, and the experiments, which will be shown in the polariscope, 

 may be taken simi)ly as a few illustrations of the subject and nothing 

 more; but they will, at any rate, be suggestive of the large field to 

 which this method of analysis can be applied. A vast amount of 

 mathematical proof can be illustrated graphically by various experi- 



