Full Idea
Husserl's 'eidetic variation' implies that we can judge the essential properties of an object by varying the properties of the object in imagination, and seeing which vary and which do not.
Gist of Idea
Imagine an object's properties varying; the ones that won't vary are the essential ones
Source
report of Edmund Husserl (Ideas: intro to pure phenomenology [1913]) by Anand Vaidya - Understanding and Essence 'Knowledge'
Book Reference
-: 'Philosophia' [-], p.820
A Reaction
The problem with this is that there are trivial or highly general necessary properties which are obviously not essential to the thing. Vaidya says [822] you can't perform the experiment without prior knowledge of the essence.