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NOTES AND CORRESPONDENCE. 



On the Aperture of Object-glasses.— It appears to me that JOUr 



correspondents on the subject of the aperture of object-glasses 

 for niicioscopes, and the methods of measuring the same, 

 have left the simple means of ascertaining tlie angle of aper- 

 ture, and taken up with such complex methods, that they 

 have been led into very considerable errors ; and hence the 

 erroneous results, in my opinion, of Professor Robinson and 

 Mr. Wenham, particularly with regard to objects mounted in 

 balsam. My method of measuring the angle of aperture is, 

 to use the object-glass of the microscope as the objective of a 

 diminishing telescope, making use of a single lens of an inch 

 and a half focus for the eye-piece of the telescope, and then 

 fixing this little telescope on a divided circle witli the focus 

 of the objective over the centre of the circle, or else by placing 

 two candles so that each of them may be at the extreme edge 

 of the field of view of the telescope. In the first case, the 

 angle of aperture is accurately measured by the circle, when 

 the image of the flame of a distant lamp or candle is made to 

 traverse the field of view of the telescope ; in the second case, 

 lines drawn from the objective to the two candles form the 

 angle of aperture, which may be easily measured by a com- 

 mon protractor. Now, by taking either of those methods, 

 and measuring the angle of aperture with nothing intervening, 

 with a slider containing an object mounted dry, or one with 

 an object mounted in balsam ; the results were (as they ought 

 to be from the laws of light) in all cases exactly the same. 

 Had Professor Robinson's and Mr. Wenham's results, with 

 regard to balsam-mounted objects, been correct, the two 

 candles placed at the extreme edge of the field of view, in a 

 lens of 150' of apeiture, would have required to have been 

 brought more than four times as near together,* when the 

 slider with the balsam-mounted object was interposed, as 

 when it was not ; but the candles did not require moving, but 

 remained at the edge of the field, whether the slider was 

 there or absent. Again, two sliders were taken of exactly the 

 same thickness, the one containing objects mounted dry, the 

 other objects inounted in balsam ; one of these being placed 

 on the stage of the microscope, was illuminated with such 



* The proportion of the tangents of 75° and 40°, or half the angle of 

 aperture. 



