2 Scientific Proceedings, Royal Dublin Society. 



case the lines appear black on a faintly illuminated ground, this 

 is called the Bright Field Illumination. 



2nd, By throwing light on the wires in such a direction that 

 none of the beam can by possibility enter the eye-piece, in which 

 case the lines appear luminous on a dark ground. This is called 

 the dark field or the Bright Line Illumination, 



The first, which can be used for all stars but those whose visibility 

 is so slight as to be interfered with by the necessary amount of false 

 light required to render the webs visible is more specially suited 

 for faint stars and nebulae, but can be used for any class of object. 



The optical arrangements by which the Bright Field Illumina- 

 tion is obtained are too obvious to require detailed description. 

 Sometimes a small, diagonal mirror is used, placed centrally in 

 the tube and illuminated by a lamp hung in gimbals so as to pre- 

 serve its verticality ; sometimes an elliptical diagonal mirror, 

 with an elliptical hole sufficiently large to admit the pencil of 

 light from the objective. But this is a matter of extreme simpli- 

 city, requiring no special care further than that the illumination 

 should be as far as possible symmetrical and not one-sided, a 

 condition easy to fulfil in this case. 



The optical arrangements, however, for producing the " bright 

 light " in " dark field " illumination are not so simple, and as 

 this is a matter in which no thoroughly satisfactory result has 

 yet been attained, we purpose describing the various arrange- 

 ments at present in use somewhat more particularly. 



One of the most common arrangements in use is to have four 

 lamps attached to the micrometer-box, directing beams of light at 

 an angle of about 45° on the wires. This plan has several disadvan- 

 tages. The angle of 45° is about the largest that can well be 

 used, and this is not sufficiently great to prevent some stray 

 light from impinging on the field lens of the eye-piece, and though 

 it does not actually enter the eye as a beam of light, it renders 

 the field lens slightly luminous and destroys the blackness of 

 the field which is essential to this class of illumination. Another 

 disadvantage is the extreme inconvenience of having lamps 

 attached to a micrometer close to the observer's eye. These 

 lamps cannot well be hung in universal joints, consequently every 

 time the telescope is turned, or micrometer revolved into different 

 positions, the lamps must be re-arranged, and if this be not done 



