230 Analyses of Books. [Sept. 



telescope ; only one image will be formed, and that will be as 

 distinctly formed as with any lens that does not possess the 

 double refracting property. 



The eye tube is so constructed that the plane through which 

 the two" images move can be placed parallel to the line in the 

 object which is to be measured ; and if this motion is furnished 

 with a divided circle, it will correctly answer the purpose of a 

 position micrometer. 



The value of the scale is found from the known diameter of 

 any distant object, and will vary in proportion to the magnifying 

 powers of the eye tube ; its value increasing in proportion to the 

 increase of those magnifying powers. 



After referring to the plate which is needful to render the 

 nature of the instrument perfectly intelligible, Mr. Dollond con- 

 cludes with observing, " When 1 constructed this micrometer, 

 it was my intention to have applied it to the measurement of the 

 angles that are subtended by the apparent diameters of the fixed 

 stars, as seen in achromatic refracting telescopes, for the purpose 

 of determining their relative magnitudes ; also of measuring the 

 distances of those double stars that would come within the range 

 of the micrometer ; but from being compelled to attend to busi- 

 ness of more immediate consequence, 1 am not able to accom- 

 pany this description with any measurements that are sufficiently 

 important to be interesting ; although I am fully convinced from 

 the trials I have made that the micrometer is quite equal to the 

 purposes for which it was intended." 



X. The Bakerian Lecture. — On the best Kind of Steel and 

 Torm for a Compass Needle. By Capt. Henry Kater, FRS. 



The experiments which form the subject of the present paper 

 -were undertaken by Capt. Kater, in consequence of its having 

 appeared, on the return of the first expedition which sailed for 

 the discovery of a "North-West Passage, that from the near 

 approach to the magnetic pole, and the consequent diminution 

 of the directive force, the compasses on board had become nearly 

 useless. Capt. Kater was, therefore, anxious that the next 

 ■expedition should be furnished with instruments, combining as 

 much power and sensibility as possible. 



After numerous experiments as to the material, form, polish, 

 and other circumstances connected with the power and sensibi- 

 lity of the needle, which it would be scarcely possible to abridge, 

 Capt. Kater arrived at the following conclusions, which are 

 given in his own words. 



" That the best material for compass needles is clock spring ; 

 but care must be taken in forming the needle to expose it as 

 seldom as possible to heat ; otherwise its capability of receiving 

 magnetism will be much diminished. 



" That the best form for a compass needle is the pierced 

 rhombus, in the proportion of about five inches in length to two 



