171 



NOTE ON THE CUT SUCTORIAL TUBES OF THE 

 DRONE-FLY'S PROBOSCIS AS A SUGGESTED TEST- 

 OBJECT FOR MEDIUM POWERS. 



Br A. A. C. ELior Merlin, F.R.M.S. 



{Read January 20th, 1905.) 



The expert has no difficulty in utilising a variety of objects, each 

 one of which may serve as a test for objectives of suitable power. 

 He judges by the quality of the resultant image taken as a whole, 

 being thoroughly conversant with its appearance under lenses of 

 the most perfect construction. 



In this manner the well-known Blow-fly's proboscis has been 

 used as an excellent and sensitive test for objectives ranging from 

 an achromatic 2 in. of N.A. *13 to the finest apochromatic ^ in. 

 of N".A. "98. For the low-power lenses, the sharp definition of the 

 structure of the cut suctorial tubes, and the general aspect of the 

 entire proboscis, form the test ; while the satisfactory rendering 

 of the minute spinous hairs may, to trained eyes, sufficiently 

 strain the defining qualities of the most powerful dry apochromats. 

 But place this object in the hands of a tyro, who has only 

 recently acquired a student's microscope and is desirous of 

 satisfying himself that the ^ in. or 1 in. usually supplied with 

 such small instruments is of fair quality, and no satisfactory 

 result will be attained ; for he will easily see the spines in question, 

 but will be able to form no decided opinion as to whether the 

 image afforded is really so good as it should be. 



Even in the hands of a novice, however, the Blow-fly's proboscis 

 may be taken as probably the simplest and most satisfactory test 

 for objectives up to the 1 in. of N.A. - 3, as any low-power lens 

 which will show the tubes and spines cleanly and crisply defined, 

 without fog, and exhibiting strong contrast, when a very large 

 solid illuminating cone is employed, must be sufficiently good for 

 all practical purposes ; but with medium-power lenses of 

 ]ST.A. # 6 to N.A. '9, which should easily grasp all the structural 

 features of this object, no one except a microscopist of considerable 

 experience would be likely to form a reliable opinion. 



