Royal Microscojpical Society. 205 



II. — Linear Projection considered in Us Application to the 

 Delineation of Objects under Microscopic Observation. 



By Chakles Cubitt, C.E., F.E.M.S. 



{Read before the Eoyal Microscopical Society, April 5, 1871.) 

 Plates LXXXII., LXXXIII., and LXXXIV. 



I HAVE frequently encountered some difficulty in the endeavour to 

 interjiret many illustrations of objects "drawn under the microscope," 

 and this difficulty has arisen from a want of coincidence in the 

 representation of different views of the same subject under the 

 varying conditions imposed by a dorsal, a ventral, a lateral view, 

 a view taken from above or from beneath, and stated to have been 

 drawn to the same scale. It has been impossible, with the par- 

 ticulars thus defectively indicated, to interpret the details with 

 sufficient perspicuity to enable me to arrange them in a consistent 

 whole, and to construct a correct drawing therefrom, the one view 

 having proved to be an incorrect representation of another in its 

 altered position ; so that while appreciating the exquisite finish in 

 the execution of the works in question, I have been disappointed 

 to find that the drawings have not accurately represented the forms 

 they were intended to portray ; and in selecting particular instances 

 as examples of such inconsistencies, I desire sincerely to express to 

 the authors of the same that my sole object is to assist them as 

 fellow-workers, and not to provoke their animosity. 



I beg permission therefore first to refer to two figures illus- 

 trating Mr. Saville Kent's instructive pajier " On a New Anchoring 

 Sponge," Dorvillia agariciformis, read at a meeting of this Society 

 on Nov. 19th, 1870, of which Figs. 1 and 2, Plate LXXXII., are 

 reproductions ; Fig. 1 being a |:)?a?i, that is to say " viewed from 

 above," and Fig. 2 an elevation of a large bi-temately terminating 

 spiculum X 20 linear ; and accepting the p/a?i as correct, we find, 

 by placing the elevation immediately beneath it, that in this view, 

 two points only, c and c, are identical with the positions of their 

 coincident points on the p)lan, which will be made manifest by 

 following the vertical fines projected from the one view to the 

 other, Figs. 1 and 2. 



The extremities of the branches c and a Fig. 2 could, in 

 their normal state, only assume the positions shown on this figure 

 under the condition of the axis of the spiculum being considerably 

 inclined ; and accepting the distance indicated by x Fig. 2 as a 

 datum or starting point, it represents the horizontal distance be- 

 tween these points indicated by x on Fig. 1, and by setting off this 

 distance x, between parallel lines as at Fig. 4, we get the angle at 

 which a plane bounding the said points must necessarily assume, 



