Quekett on the Microscope, ed. 1855, p. 497. The microscope, with its objective 

 and eyepiece as in ordinary use, is placed horizontally, a candle is set on a level 

 with it, a few yards distant ; the microscope is then turned, till, on looking 

 through the eyepiece, the field of view is bisected, half being light and half 

 dark. The microscope is then turned round, with the focns 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, pre- 

 ferably, 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. 3Jic. Sc, 1854, p. 134. A 

 lens of about 3 in. focus being placed centrally, in a sliding cap, above the 

 eyepiece, the image of the flame can be observed, and the angle measured 

 with great accuracy; also the condition of the definition at the margin of 

 the field can be ascertained, sometimes suggesting the utility of reducing 

 the angle of the objective. This plan appears 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 eyepiece is replaced by a cone having a small aperture, through 

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

 centre upon 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 employment 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 eyepiece and objective, 

 and intercepted 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 3rd Vol. of the Quart. 

 Journ. Mic. Sc, 1855, p. 85, which he considers simpler than Mr. Wenham's. 

 He does not use the Huyghenian eyepiece, but a lens of 1£ in. focus, " as 

 the eyepiece of a diminishing telescope." Two candles 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 oblicmely, a distorted 

 image of the candle may be seen at a greater angle than that which is en- 

 gaged 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 em- 

 ployment of a combination of two lenses instead of a single lens, the 

 instrument being placed 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 eyepiece, which amounts to a re-invention 



