PROGEESS OF MICROSCOPICAL SCIENCE. 237 



half being liglit and lialf dark. The microscope is then turned 

 round, with the focus of the objective as a pivot, until the opposite 

 half of the field is illuminated. The angle can be measured by lines 

 drawn on a suitable part of the instrument, or, preferably, by a 

 divided semicircle. This method answers very well up to 90° or 100°, 

 but for larger angles is not nearly so accurate as that devised by Mr. 

 Wenham, and described in the ' Quart. Journ. Mic. Sc.,' 1854, p. 134. 

 A lens of about a ^ in. focus being placed centrally, in a sliding cap, 

 above the eye-piece, the image of the flame can be observed, and the 

 angle measured with great accuracy ; also the condition of the defini- 

 tion at the margin of the field can be ascertained, sometimes suggesting 

 the utility of reducing the angle of the objective. This plan ajipears 

 to have been used some years earlier by Amici. In the ' Quart. 

 Journ, Mic. Sc.,' 1854, p. 293, Mr. Gillett's method is described: 

 this was communicated to the Royal Society on March 9, 1854. The 

 eye-piece is replaced by a cone having a small aj^erture, through which 

 light is sent. The objective is focussed on an object which forms the 

 centre ujjon which a second, or examining microscope, attached to a 

 divided arc, turns. This plan is described in Mr. Hogg's ' Treatise 

 on the Microscope,' 1871, p. 45, as " a very perfect instrument," but 

 there seems to be some source of error connected with the employ- 

 ment of a second microscope. Professor Robinson's method was first 

 brought before the Royal Irish Academy in 1854, and is described in 

 the ' Quart. Journ. Mic. Sc.,' 1854, p. 295. Rays nearly parallel 

 are sent through the eye-piece and objective, and intercej)ted by a 

 screen at a distance greater than the focus. This distance, and the 

 diameter of the base of the cone of rays so formed being known, the 

 angle is easily calculated. This is a very elegant method, and likely 

 to be valuable in certain disputed cases as to the true angle of immer- 

 sion lenses. Mr. Sollitt describes a method, in the third volume of the 

 ' Quart. Journ. Mic. Sc.,' 1855, p. 85, which he considers simpler 

 than Mr. Wenham's. He does not use the Huygenian eye-piece, but 

 a lens of 1^^ inch focus, " as the eye-piece of a diminishing telescope." 

 Two caudles are employed, and moved till their images are seen at 

 the extreme edges of the field. This is described in ' Carpenter on 

 the Microscope,' 5th ed., 1875, p. 202. It is open to the objection 

 that if the observing lens is held obliquely, a distorted image of 

 the candle may be seen at a gi'eater angle than that which is engaged 

 in forming the image of the object, and probably the angle is over- 

 stated. Mr. Wenham's (or Amici's) method seems to have been again 

 re-invented, as it is attributed by Mr. Brooke ( ' Quart. Journ. Mic. 

 Sc,,' 1864, p. 84.) to Professor Govin, of Turin. It was used in the 

 examination of objectives at the International Exhibition of 1862; 

 the only differences were the employment of a combination of two 

 lenses instead of a single lens, the instrument being j^laced in a 

 vertical instead of a horizontal position, and strips of white paper on 

 a dark cloth used instead of candles. In the ' Quart. Journ. Mic. Sc.,' 

 vol. vii., p. 256, Mr. Peter Gray examines the images of two flames in 

 the objective without an eye-piece, which amounts to a re-invention of 

 Mr. Sollitt's method. Mr. Stephenson, adopting the same system, 



