Angular Aperture of Ohjed-glasses. 113 



not from ill-temper, like the last, but it is apt to offend. I find it 

 difficult to restrain the propensity at times, though quite aware 

 how few can accept it. Again, there is the meek and amiable 

 style, that can neither make or answer a strong objection, saying, 

 " I am sorry that I have disturbed you, gentlemen ; I will drop 

 the question rather than disagree !" This I cannot take credit 

 for, but judging from the attitude of some of my opponents, it 

 would seem as if I am a wolf amongst the lambs. 



This prologue may afford material to those who find it more 

 easy to comment upon words than scientific facts ; it, however, 

 precedes an announcement that I have to make, that object-glasses 

 have at length arrived at 180^ of aperture. A country corre- 

 spondent in this Journal once asked the question whether " the 

 old science of optics was now played out, or if the world has 

 taken a turn the other way for a change ? " * Can this be ? a 

 position at which simple teaching has told us nothing can be seen, 

 and yet here is a thing said to be seen where it cannot exist. I 

 had the glass in my hands, and rubbed my eyes in vain, but there 

 it was, beautifully engraved, " E, B. Tolles, Boston. Immersion 

 |th, 180° balsam, angle 98°." Yes ! ONE HUNDKED AND 

 EIGHTY DEGKEES ! 



Having recovered my surprise, I proceed to tests. My remarks 

 have only to refer to the question of aperture. The object-glasa 

 belongs to Mr. Crisp, a gentleman well known for his liberality and 

 freedom from prejudice in the pursuit of microscopical science, and 

 it is understood that the maker or sender is willing that this 

 peculiar property of aperture should be commemorated. 



The object-glass was first tried on such tests as required large 

 aperture for their determination. I could ghmpse striae of equal 

 difficulty on the same object in Moller's proof slide, with another 

 object-glass of 120°. Is all aperture beyond 120° then useless ? 

 I next measured the Tolles ^th in the ordinary sector with lenses 

 quite closed. Light was seen up to 180°. There was no definite 

 margin at any point, for disappearance was gradual. Then came 

 the question of how much of this light belongs to aperture in 

 relation to diameter of front lens and focal distance. 



The diameter of front lens to edge, as measured by micrometer, 

 was "043 of an inch. If anyone had asked me what diameter 

 would be required for 180° ? not wishing to exaggerate (as some 

 might argue that even space must have its limit), I should have 

 said not less than the crown of your hat. Therefore a diameter 

 of "04:3 for an angle of 180° is a marvellous accomplishment. If 

 1 had also been asked what focal distance could be got with 180°? 

 my answer would have been • 000, but the actual distance in this 

 lens was "OlS, a most comforting length for 180°. 



* 'M. M. J.; March, 1873, p. 124. 



