18 The Ojptical Quality of Mr. Tolles' ]^ Objective. 



method I judge, and it is this — for a strict test of angle let the 

 objective be measured in position as used on the object. 



Mr. Wenham having adopted the semi-cyhnder, I will remark 

 that this can be very easily done with that, in all cases ; but as 

 an effectual means to dismiss any suspicion of false light, " not 

 going to form images," let this, which he has well approved, be 

 done, viz. put light doivn thro ugh the tube of the microscope. 

 Then if a low eye-piece be put in place in the body, and a beam of 

 sunlight be the light used and directed through the eye-piece down 

 the body along the axis, it will have emergence at the cylindrical 

 surface, and, if intercepted by a band of thin moist paper adhering 

 to the same surface, the circular houndary of the emergent pencil 

 will be sharply defined, and the angle can be accurately and 

 instantly read there. 



Of course, if a dry mount is used, only 82° can appear emergent 

 at the surface when the air-angle of the objective is infinitely near 

 to 180^, and reference index of cylindrical piece being 1 '525, as 

 Mr. Wenham used to stipulate /or halsam, with a view, it is under- 

 stood, to simplify the argument, its index varying slightly from that. 



Whatever the angle read at the cylindrical surface, the corre- 

 sponding air-angle is readily known or deduced from it ; for surely 

 it will not be claimed that other than " image-forming rays " will 

 have emergence from the objective-front when derived from a beam 

 of unmodified sunhght entering the eye-piece. 



And now, though claiming for that method that it is safe 

 against false light, yet I admit (or claim) that sunlight through the 

 eye-piece and thence through the objective is not necessary for 

 accurate results ; not at all ; for I have always found the results of 

 the reverse method to nicely correspond — ivithout any diaphragming 

 of the field, such as Mr. Wenham describes in the ' Monthly ' for 

 March last, it will be found that the limits of angle are the same, 

 whether the light be put down through the body, or reversely, the 

 light has direction from a radiant in front, and angle-limits 

 observed at the eye-piece. 



There is especial convenience, however, in the latter method, 

 and having happily Mr. Wenham's endorsement of the semi- 

 cylinder for such purpose, I will j)articularize. Thus if the 

 cylindrical piece, balsam-mounted-object thereon, and objective be 

 in position, and the observer having the object in plain view 

 through the microscope, then, with lamp in hand, while moving 

 the screw-collar forward or back for the limit of angle as the 

 radiant is moved out from the axis, the focal adjustment being 

 maintained consentaneously, the real maximum angle will be 

 ascertained by working for the greatest obliquity of the illu- 

 minating ray that bisects the field. When that greatest obliquity 

 of incidence is gained, measuring the angle can be attended to. 



