288 PHILOSOPHICAL TRANSACTIONS. [aNNO 1760. 



this focus, it is scarcely possible to avoid touching the object with the glass, if it 

 be not placed between laminae of talc or isinglass ; and if it be so placed, even 

 the thinnest talc bears a considerable proportion to this 576th part of an inch 

 and will prove an unsurmountable obstacle to the seeing any object, unless by 

 some very happy accident. The other globules, whose focus is not quite so 

 near, are liable proportionally to the same inconvenience. 



Thevery great powerof magnifying glassglobules is sufficiently well known: many 

 years ago they were much used, and highly boasted of on that account. But they 

 now, long since, have been laid aside, and convex lenses substituted in their 

 room ; and that with very good reason, from the difficulty in the application of 

 such globules, from the deficiency of light, from the distortion of the image seen, 

 from the painful straining of the eyes, and from the boundless latitude given to 

 imagination and conjecture, for want of sufficient distinctness and precision. 

 Nothing can be more injudicious than the desire of such excessive magnifying 

 power: whenever we can see an object clearly and well defined, we ought to be 

 contented; all beyond this there is no dependence on. In some letters, sent 

 with these glasses, the society has been favoured with uncommon observations 

 on the globules of the blood, described as having been viewed (it is not said by 

 these glasses) floating in the serum, and sometimes changing their figure: and also 

 with a long account of the impregnation of vegetables; where we are told, 

 that the exquisitely minute corpuscles or seminal particles, emitted by the grains 

 of the farina foecundans, have been seen to enter into, and be conveyed along 

 tubes exceedingly small, which at the time dilated and contracted occasionally 

 to convey them to the ovarium.* Mr. Baker was extremely desirous to repeat 

 these experiments: but as it was absolutely necessary to spread the blood as 

 thin as possible, to render it very transparent, without which nothing can be 

 seen by such small glasses, he could not possibly prevent its becoming quite dry, 

 before he could apply it to the eye, and consequently was unable to perceive 

 any floating globules: and though he has been many years conversant with 

 microscopes, he has not been able to contrive any method of applying the parts 

 of generation of plants in such manner, to these glasses, as to view this 

 wonderful impregnation. 



It is, however, proper to take notice, that in these letters an apparatus is 

 described, to be added to Wilson's microscope, when these glasses are made use 



* It must be observed here, injustice to Mr. TurberviUe Needham, r. r. s. that he was the person 

 who first discovered, that, on applying water to the farina foecundans, many of its grains emitted 

 streams of exquisitely minute globules, as if through a small ajjerture: this he published in the 

 year 1745, and from thence imagined the impregnation of plants to be carried on in a manner 

 somewhat similar to that in the account referred to; but the same justice must allow, that before 

 Father di Torre, nobody is supposed to have seen these several progressions towards impregnation.— 

 Orig. 



