160 HOW TO SEE WITH THE MICROSCOPE. 



has used several of these instruments ; the first one wa? 

 made by Mr. Tolles, the angle of facet being 26 An- 

 other one made some little time afterwards had facet of 

 18. Subsequently Messrs. Spencer have made him sev- 

 eral having angle of 15. The genuine instrument, as 

 made by Mr. Wenham, is only suitable for use with 

 objectives of high balsam angle, while those of the 

 modern form can be employed with more or less suc- 

 cess in conjunction with glasses possessed of tolerably 

 wide air apertures. We have spent a great deal of time 

 in the study of this instrument, including the several 

 patterns named, and we find that when used with glasses 

 of the highest balsam apertures, such as are made by 

 Messrs. Spencer or Mr. Tolles, and over the severest 

 tests, such as amphipleura pellucida, the resolutions are 

 quite as strong and satisfactory when the illumination is- 

 obtained from the modified " reflex" as when the instru- 

 ment described by Dr. Carpenter is selected, while the 

 latter has the serious disadvantage of being adapted for 

 use over balsam mounts only. 



The instrument can be adapted to almost any good 

 and reliable stand, even should the latter not be pro- 

 vided with centring apparatus; yet if the sub-sta^e 

 collar be itself well centered, a little ingenuity on the 

 part of the observer will secure good results. It is, 

 however, a great convenience, when using the " reflex," 

 to employ a stand allowing the sub-stage to approach 

 the slide or to be drawn away therefrom. 



To use the " reflex," place it in the sub-stage fitting, 

 select a low power, say a half-inch or a two-thirds, and 



