C12 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



The intensity of liglit, taking that of the 3-lino objective as = 1, 

 is for the objectives of 6, 12, 18, 24 lines focal length, 0-53, 0-25, 0- 144, 

 0*091 respectively. 



With screw Microscopes, objectives of less than 8-15 lines focal 

 len<^th have rarely been employed, in spite of the advantages which they 

 would realize. It is advisable with the strongest objectives, and even if 

 possible with the others, to use orthoscojiic eye-pieces which give greater 

 definition of image near the borders of the field. 



To test the relative advantages of short and long focal length in the 

 objective, a comparison was made between a Microscope of the author's 

 construction, and a theodolite Microscope, with an objective of 30 mm. 

 focal length, 6*7 mm. free ajierture, and (as it stood 38 'G mm. from the 

 scale) 10° angular aperture. The magnifying power was 40 (objective 

 3-5 and eye-piece 11*3). The total length from the scale to the end of 

 the eye-piece was 20*5 cm. The other Microscope had a length of 

 95 mm. from the scale to the end of the eye-piece, magnifying power 

 = 50 (objective 3*6, eye-piece 14), focal length of objective = 5 lines, 

 angular aperture = 20°. It was found that with the smaller Microscope 

 the intensity of light was three times as great as with the larger. 



There are cases in which for special reasons long Microscopes arc 

 desirable or necessary, as with dividing machines where the heat of the 

 body is to be avoided, or where it is necessary to read from a distance. 

 The angular aperture may here be increased by using an objective com- 

 posed of two -weaker lenses of greater diameter so as to gain light ; or 

 the tube may be lengthened by a terrestrial eye-piece (with erect image) 

 without weakening the objective ; or the light may be increased by 

 settinf^ the Microscope at an angle to the plane of the scale. This last 

 contrivance, which is so convenient with vernier lenses, can only bo 

 applied to Mi<;roscopes to a limited extent. The inclined position serves 

 to reflect light from the silvered scale into the lens or Microscope ; so 

 that the divisions appear as sharply defined black lines upon a bright 

 white ground. In the normal position of the Microscope, when it is 

 perpendicular to the scale, the angles of incidence and reflection must 

 both be 90°, i.e. the light must come vertically downwards; this is 

 effected by the illuminator. If the Microscope is inclined backwards 

 the field is brighter, but the divisions are not visible in their whole 

 length, but only in a small part. In practice, however, a backward 

 inclination of 10° may be attained ; the light incident between 80° and 

 90° is then reflected from the scale directly into the Microscope and 



