ASTRONOMY. 211 



It will be useful to kuow, also, if the telescope is able to show the solar 

 gninulrttioDS ou any favorable opportunity, and also the degree of 

 visibility of these granulations during the transit. 



Art. 2. It will be weU to employ a reflecting prism, or a polariscopic 

 eye-piece, to diminish the heat and consequent danger to the observer's 

 eyes. 



If it be decided to use a silvered objective, a method which offers the 

 great aJVantage of eliminating all the obscure heat rays and doing 

 away with errors from distortion arising from heating of the interior of 

 the tube, the excess of light may be absorbed by a neutral tint glass 

 composed of two glasses of similar thickness, one being colored and the 

 other colorless. 



Art. 3. The eye-pieces should be positive, achromatic, and of a i^ower 

 of 150. The observations of contacts should be made in a field suffi- 

 ciently clear to show, plainly projected on the solar disc, two wires sej)- 

 a rated by a distance of 1". 



Means should be employed to remove as far as possible the effects of 

 atmospheric dispersion. 



The setting-point of the reticule should be previously ascertained on 

 the stars, or by means of a collimator focused to stars. 



In cases of observation by projection, corresponding means should be 

 in ployed. 



Art. 4. The times corresponding to internal contacts may be defined 

 as follows: 



Ingress: The moment when an evident and, at the same time, per- 

 sistent discontinuity in the illumination of the apparent limb of the sun 

 jointng the point of contact with Venus disappears. 



Egress : The moment of the first appearance of an evident and, at 

 the same time, persistent discontinuity in the illumination of the solar 

 limb joining the point of contact. 



If the limb of two stars coming into geometrical contact, without ob- 

 scuration or deformation of the interposed thread of light, the instant 

 previously defined is that of contact. 



If there be produced a black drop or ligament, well defined and as 

 dark as the body of the planet, the precedingly defined instants are: 

 for ingress, that of definite rupture; and for egress, that of the first ap- 

 parition of the ligament. 



Between these two extreme cases, other appearances may bo pro- 

 duced when the instants of contact may be noted as follows : 



If, the limbs remaining without deformation, there is produced an ob- 

 scuration of the luminous thread, without the shadow, however, being 

 as dark as the body of the .planet, the observer notes the instant of 

 geometrical contact. The moment of the formation or disappearance 

 of tills shadow should also be noted. 



If the shadow is aim ost or becomes quite as dark as the planet, the 



