Microscopical Essays. yy 



The object may be viewed either with or without the guide for 

 the eye ; a fmgle obferver will fee an object to the greateft advan- 

 tage by ufmg this guide, which is to be adjufted as we have 

 defcribed, page 75. If two or three wifh to examine the object 

 at the fame time, the guide for the eye mull be laid afide. 



Take the large lens out of the groove, and receive the image 

 on the grey glafs ; in this cafe, the guide for the eye is of no ufe ; 

 if the grey glafs is taken away, the image of the object may be 

 received on a paper fcreen. 



Take out the grey glafs, replace the large lenfes, and' ufe the 

 guide for the eye ; attend to the foregoing directions, and adjuft 

 the object: to it's proper focus. You will then fee the object in a 

 blaze of light almoft too great for the eye, a circum (lance that 

 will be found very ufeful in the examination of particular objects ; 

 the edges of the object in this mode will be fomewhat coloured*, 

 but as it is only ufed in this full light for oceafional purpofes, it 

 has been thought better to leave this fmall imperfection, than by 

 remedying it, to facrifice greater advantages ; the more fo, as this 

 fault is eafily corrected, and a new and interefting view of the 

 object is obtained, by turning the inftrument out of the direct rays 

 of light, and permitting them to pafs through only in an oblique 

 direction, by which the upper furface is in fome degree illumi- 

 nated, and the object is feen partly as opake, partly as tranfparent. 

 It has been already obferved, that the tranfparent objects might 

 be placed between the flider-holders of the flage for opake ob- 

 jects, and then be examined as if opake.. 



Some; 



